That strangers are going to meet and fall for each other seems so fucking unlikely that we should all assume that neither party will call or write.  If the other person is utterly thrilled and calls/writes, then the other (noninterested) party can call/write back and say "no thanks."

Indeed, why bother.  In fact, you most likely decide you're uninterested in a person during -- rather than after -- the first date, so why not simply up and leave without a word once you've reached that conclusion?  Such economy of expression would undoubtedly fill your would-be suitor with gratitude for having wasted as little of his time as possible.

I was also of the mindset that a note was in order either way, but have my doubts now.  If it's only been one date and things are just kind of so-so, is it necessary?  I can see if perhaps there have been several meetings, a polite note is the right way to go, but it should be short and sweet and really requires no lengthy explanation of what one party may perceive the differences and obstacles to be.  

Even after a lengthy relationship, if you haven't talked it all out already, what's the point of getting wordy at the end?

In this circumstance, the gentle lie suffices.  If the date was generally pleasant but devoid of chemistry, I put the burden back on me:  "I really enjoyed your company and _________(insert personalized compliment), but my own lack of spunk and charm reminded me it's too soon to start dating after my last lousy relationship. Thanks again for the__________(insert dinner/drinks/etc.) and here's hoping you run into a gal who's got her act a little more together."  

If the fellow was rude/crass/or crazy, a simple "Fuck off" is generic enough not to sting for life, but clear enough to thwart further exchanges.  

There should be a box you can check which states: "I swear I'll be fine if, in fact, you don't dig me, and you swear you'll be fine if, by chance, I don't dig you"

There can be a subcategory for the hard-core, wherein you promise to let the other person know the moment it becomes hopeless, even if it is within seconds/minutes.

if you go for a job interview--and you know it went not so great--do you still follow up with a thank you note? think about KARMA!

I do believe that saying no thank you is better than silence--in any situation.

Your advice sucks, bart, but thanks anyway.

I have never written "Fuck off", but I have said it face-to-face.  Upon learning a date would not conclude with sex, an arrogant suitor became denegrading and began ranking me in contrast to "several other women I'm currently sleeping with."  Apparently my body was "decent" and my face was "beautiful in some angles but revolting in others."  I said, "And yet, despite the fact that I fall so below your high standards, you are out with me and have failed to seduce me.  FUCK OFF!"  (Less cool part of the story:  What followed was a silent truck ride to a parking lot where he dumped me and I had to shuffle four blocks to my car in tight pants.)  

anytime

I think silence is appropriate until the other party issues a second invitation.  Before then it can be presumptuous.  I think it happened once or twice where I thought the date had gone nowhere and I had no intention of asking her out again, and I was surprised to receive a note saying she didn't want to go out again.  I was sore tempted to write back, "What makes you think *I* wanted to go out again with you, either?" but thought that would sound whiny and sour-grapesy, and so just didn't respond.

Dvaleriey, when you said "denegrading" (not a word) did you mean "degrading" or "denigrating"?

Because ranking was involved, I imagine "degrading" is correct, but it implies a lowering of status I won't cop to.  Denigrating is the work I meant since the act of defaming seems more desperately one-sided.  

PS  Did we have a date that didn't end in sex?  

It depends on the situation.  If it's a first date and you aren't really all that interested I see no reason to write to a person just to let them know that.  If the other person writes to you and is letting you know that they would like to go on another date, then I think a thanks-but-no-thanks note is in order.  It's just rude to not respond at all.