I know next to nothing about photography, but I want to get into it. I have regular digital cameras, but want to upgrade to a SLR. Can anyone recommend one in particular, or tell me what specs to look for. Would like to keep it under $500, but I am relatively flexible. I would use it mostly for shooting on vacation. Thanks in advance.
Almost anything that Canon or Nikon makes in a DSLR won’t disappoint. Are you including a lens in that budget?
In that price range your choices are fairly limited unless you go secondhand.
As in BH photo lists 8 cameras under $500, vs 25 for 500-750.
I would suggest choosing between Nikon and Canon, simply because they have the largest current lens ranges, and the most third party lenses. While for now you may only want a couple of lenses, you never know what you might want in future, and the more options the better in my view.
The options at that price range are the Nikon D5000, D3100, and the Canon T1 or T2, depending on what end of 500-750 you look at. You really cant lose with any of them in my view, and its best to just see what feels best when you hold it and muck around with the controls etc.
The other option would be to consider the mirrorless cameras ie the Olympus Pen, Sony NEX etc if you find a viewfinder a pain in the neck to transition to.
Otara
Everyone was very helpful to me over here. I had a similar request.
My dream camera is the Canon T2i, but it is over your price. Can do full 1080p video and takes nice stills fast with great resolution and low light performance.
Just for the body. Honestly, I have no idea what these things cost. What kind of lenses would I want/need? I could probably swing the Canon T2i. Is that much better than the T1i? Thanks.
The main improvement is with video, but the T2i got very good reviews in general, ie good image quality and features for its price bracket, the sensor it uses is basically the same as the Canon 7D, a far more expensive camera.
Lenswise, it really depends what you want to shoot. The standard kit lens 18-55mm IS (not 17-55mm, thats much more expensive) is surprisingly good, after that it depends if you want to do long distance as well, or wide angle etc. For long range theres a kit lens for that too, the Canon 55-250mm IS, which is great bang for the buck.
Theres better of course, but they will cost substantially more too.
Someone else would need to cover Nikon, but theres two pretty similar good quality kit lens choices from memory.
Otara
I own the T1i, and I lust after the T2i. Higher resolution still and video, higher ISO, and takes SDXC cards, which will be standard in a few years. They hold much more capacity.
I’ve just bought a Panasonic Lumix G2, and a 14-140mm lens to go with it. Nice small SLR with some clever optical magic to reduce its size, really well featured and great pics - and it does HD video too. The weight of the lens kind of negates the compactness of the camera, and it isn’t interchangeable with other camera lenses without an adapter, but it is a really really nice system to use. Not cheap, but not hellishly expensive either.
It’s pointless to price out a camera package without including the lenses you plan on using. The kit lens for many of the the basic packages aren’t bad; the 18-55mm lens in the Canon kit is quite acceptable to start, but at some point you’re going to want to get more lenses. That’s one the main points of DSLRs, using the right lens for the right job. And lenses cost more than camera bodies, at least really good ones. 
What do you want to do with the camera?
Some suggested reading:
Digital SLR cameras on a shoestring
CNET Digital camera buying guide
dpreview Digital SLR buying guide
Also, which cameras are you currently using, and what additional benefits are you expecting to get from a digital SLR camera?
For vacations, take a look at something light, compact and versatile, like a compact megazoom, sometimes called travel zooms.
As an ex-journalist who carried a camera every day, I can say that Nikons tend to be a little easier to use, but Canon has a better video system. I always had to try to figure out how to do something on a Canon, but the Nikon menu system and button placement always made sense to me.
I have a D40 I bought a couple of years ago (since discontinued) that is the perfect entry-level DSLR. Looking at the D3100, it looks like it’s supposed to be the replacement for the D40. The 18-55 kit lens that comes with the D3100 is the same lens that came on my D40, and it’s a handy little lens.
One thing that most new DSLR shoppers overlook is how the camera feels in your hands.
Before you make the final decision, get yourself to a camera shop where you can hold the camera and mess around with it a bit.
Often, some models ‘fit’ better than others, and that is no small consideration.
mmm
(who is 100% satisfied with his Nikon)
Exactly. I know exactly where every button is on my D40, and when I was slinging a boat-anchor D1 around, I could find everything on it without looking. When I had a Rebel in my hand, however, I was constantly having to hunt for buttons.
I took your advice and got the T2i. It came with a 18-55mm lens, and I got a 55-250mm IS lens as well. Do you think I need anything else?
Lens Filters.
Daylight.
Polarizing.
Photography book. I really liked the National Geographic Field Guide, but there are many to choose from.
Google Picasa for photo downloading and management.
Some light and your eye. Have fun 
What do you want to do with it?
Tripod
I bought it mostly for family photos, and my upcoming trip to Japan (for a wedding). I live in DC, so I plan to test it out by taking pictures at the National Mall. Nothing specific I guess.