…but warm beer can stay warm. Is there some chemical reaction which happens to it when it gets cold, making it spoil if it warms again? I have a strong suspicion that this question has been answered already but I couldn’t for the life of me find it.
It’s a chemical reaction, but is not “spoiling” or additional fermentation. Here’s a good synopsis on the effects of sunlight and excess oxygen:
“SKUNKING
This phenomenon, also known as the Heineken Effect (at least by me), is the result of a photochemical reaction between sunlight and hops. The end result is traces of the same chemical a skunk produces. Yuck. Beer packaged in green or clear glass will skunk in as little as a minute in direct sunlight. Brown glass affords much more protection, but is not a perfect shield against skunking.
OXIDATION
Heat and age will oxidize beer, giving it a lifeless, wet-cardboard like flavor. This is the leading cause of bad beer among microbrews. Avoid old and dusty bottles of beer sitting on your retailer’s shelves. More than likely, they have reacted with oxygen to give these off flavors.”
The rest (and some good polls on beer) can be found here:
http://www.epinions.com/content_14645628548
You don’t need to keep chilled beer cold. It can “warm up” without any ill effects as long as you keep in it a dark place. Preferably a cool dark place, but even room temperature won’t do it any harm. Keep it away from sunlight, though; beer hates the light. And don’t leave it for too long–any beer, chilled or not, has a reliable shelf life of only about three or four months, maximum.
I used to work in a beer store. All our stock was chilled in our store–nobody ever bought warm beer from us, because we didn’t sell warm beer. Many customers appreciated the chilling, but you can bet they didn’t run directly home and fill the fridge. On hot summer days, customers would buy a case, put it in the hot car trunk, and go off to do their other errands. And we heard no complaints.
And from my experience (taking a chilled case home at the end of my shift and leaving it under the kitchen table at room temperature for the next week), nothing happens if chilled beer warms up to room temperature.
Are we talking REAL beer here, or that American stuff? 
Walrus
I see you haven’t been here for awhile.
About 10 years ago, due to a sweet change in our alcohol laws that prohibited brewing small ammounts of beer for commercial sale, many many microbreweries popped up that make fine beer of a quality that rivals the best beer from the rest of the world.
Ya ever had a good Blonde Dopplebock, or Coffee Stout, or
a sweet Irish Ale?
It ain’t just Miller/Bud/Coors/name-other-swill-here anymore!
I bet you could find something you like 
i had some amazing stuff in California last year. it was a Russian stout i think, it was about 9% alcohol and was very good. anyone know it’s name?
also there was one called Red something about 6 or 7% i think which was good too.
Niobium Knight
A Russian Stout you say?
Do you remember the name of the brewery?
The name of the bar?
The city you were in?
A number of microbreweries make that style of beer, but since I live in Wisconsin and not California (only a couple/three thousand miles away) I can’t help without more info.
Actually, if you go on Google and enter:
California microbreweries Russian Stout,
it kicks out a whole bunch of stuff.
Perhaps you can find your beer there.
Imperial stout is a style of especially strong stouts that originated in England and were made for sale to the Czar’s court in Russia. It’s been revived by some microbreweries.
A pretty popular one, with a Russian themed label is Grant’s Imperial Stout. Just a WAG, but that might be the one you had. Though it’s made in Washington state.
Red Tail Pale Ale by the Mendocino Brewing Co. maybe? That’s one of my all-time favorites!
Or could have been Red Hook IPA. That one’s about 6.5%, but it’s not nearly as tasty as Red Tail.