If I allow my email address to be shown, can unregistered people access it, or only members?
I think anyone can see it. I just logged out and I was still able to access e-mail addresses. I had to log back in before I could reply to this thread.
You can always turn off the name in your Profile.
If you want more privacy, but still want to be available, you can get a free web-based mailbox that forwards to your regular box. If you start getting ads at that name, close it and start a fresh one. I had to do that when I got onto one financial board (The Motley Fool) that swamped you with their own multiple newsletters and bonus coupons.
Mail at most boards used to be very visible. The various board software at one time let you could click on the Members list and they would all be there in a neat table. Then the various boards complained and they made it one step removed; you have to click on each name to see the link.
There also used to be a simpler manual way of finding names - you could hover over the Mail link under a post.
Now that too requires opening of a new window.
You could always manually alter your email address to discourage “automatic” email address harvesters. Use something like smokeNOSPAMPLEASE@hotmail.com (or whatever). Most users will know to remove the “NOSPAMPLEASE” section.
Or just ignore spam, it comes with the territory. Ads are half of the visible internet anyway. You read magazines and mannage to skip the ads, so what’s the dif? Without ads the internet would still be the Arpanet, for science labs only. If you can’t step over the cow patties, you’re too young to leave the ranch house.
That was advice for YOU. Me, I got a second “profile” on the same account for friends to reach me. Goes to a separate folder.
By the way, it’s up to you to keep your email address current in your profile. If the administrators need to get in touch with you for any reason, we CAN look at the email address. If the mail bounces, we’re likely to remove posting privileges until the person gets in touch with us with a valid email address.
The button that asks if you want to allow administrators to send you email does not prevent us from sending you PERSONAL email. That button is in case we need to send out email to everyone who will allow us to. For instance, we got hacked quite a while back, and someone had everyone’s password and email. Since some people use the same password for EVERYTHING (which you really shouldn’t do, but I can understand the temptation) we needed to let people know to change their passwords.
We don’t use this information to send out spam, in case you were worrying. And as far as I know, we don’t sell this list to anyone, either, though it probably WOULD pay for a new server (and maybe even a second coffee mug for all the staff). I voted early and often against such measures, thinking that it would alienate our readers. If anyone’s gonna piss off the TMs, it’s gonna be Cecil. Or possibly me. 
Lynn
Administrator
For the Straight Dope
You’re to nice to piss anyone off. You never piss me off. And I have a very thin skin…
CatBiker, you’re too BIG for me to risk pissing you off! I have a very high regard for MY thin skin!
Besides, I happen to know you’re just a sweetheart.
Tech-Mex, ignoring spam is akin to ignoring someone calling you collect to give you a sales pitch: You can block the pitch itself out, but you still have to pay for it. People pay for their email accounts as part of the price of having internet service. Having ads sent to an account you pay for is a clever way the marketers have of forcing you to pay their marketing costs. If this was done in a more obvious way, like the collect-call scheme above, people would be up in arms. As it is, nobody quite grasps how badly they are being screwed by spammers. I have a link for you here that explains the problem in more depth. Paul Lutus wrote a page on spam here. If you want to debate this further, open a GD thread. I’ll be there. If you do not know who Paul Lutus is, hie thee to http://www.arachnoid.com and be enlightened.