I've read the bicycle passing rule - but one thing's still not clear

I guess this is the most likely scenario of what I was getting at.  With the rule stating you have to pass within 15 seconds, and with so many new racers expected in my triathlon, isn't it likely that people will overcommit to what they're capable of?  Isn't it to be expected that someone will spend more energy than they really have available in order to make a pass, get in front of the other rider, and then slow way down?  And if so, does that really require the passed rider (who might have just been maintaining a steady effort) to reduce his effort even more to drop back 7 meters before he resumes his steady effort and repasses?

If you don't want to have to drop back, don't get passed. 

The answer to your question is, yes, you must ALWAYS drop back 3 lengths before re-passing or risk a penalty.  It may not always be called, and I often see it broken in races, but it is the rule.

Exactly (:

 Which is helpful advice, really. ;-) 

I think the number of answers like this has also answered the question.  So even if someone passes you and then proceeds to slow WAY down immediately, you also have to slow WAY down to drop back before re-passing them.  That's clear.

So would passing and then slowing way down be considered bad form?  Or does it happen and it's viewed as just part of racing.

If it's done on purpose, then surely it is bad form. But probably it is mostly done out of ignorance (of one's own ability). In running, inexperienced runners do this all the time (although the consequences are far less severe) -- they blow by you at the base of a hill, then maintain the ideal line (the one you want to run except their rear end is in your way) while gradually slowing down, so that you have to go around them to pass. Yes, it is annoying. It's the price you pay for letting inexperienced cyclists like me join your race.

When does ignorance turn into bad form? Well, I guess if someone habitually does this and fails to learn, then eventually he or she becomes officially annoying and should know better and deserves a dope slap. If a person makes the mistake once or twice and then learns from the mistake, well, as I say, one of the great things about this sport is that anybody can join, and in that case, this sort of thing will happen.

EDIT:  Now, of course that's my uninformed and inexperienced opinion.

Pardon my confusion, but I have the WORST possible background on this (I used to race many years ago, so we ALWAYS drafted right up against the wheel).

It seems the key to avoiding the problem is to not be "overtaken" (don't let the front of his/her wheel get past yours), at which point the person has to either pass or fall back within 15 seconds, yes?

If you get passed, how fast do you have to "immediately" move to the rear and out of the drafting zone?

Once the person enters your draft zone from behind, the only way he can legally exit your draft zone is to pass you. He has fifteen seconds to do so. He cannot legally "fall back" out of your draft zone.

It's never been specified to me, but I assume fifteen seconds.

So, rider A attempts to pass rider B by entering the draft zone of rider B. Rider B speeds up and rider A cannot pass in 15 seconds. In fact, rider A cannot pass at all. Rider B drops his , thereby causing rider A to "fall back" out of the draft zone. Is this illegal on Rider A's part?

Second scenario; rider B does not drop rider A, however rider A cannot overtake rider B in 15 seconds. They are now going the same speed, and rider A cannot clear rider B's front wheel. Who is supposed to do what?

 Yep. Don't enter the draft zone unless you are sure you can pass.

Rider A has fifteen seconds to overtake Rider B. Again, don't enter the draft zone unless you are sure you can pass.

In either case, if Rider A cannot pass within 15 seconds after first entering the draft zone they are subject to a drafting penalty.  Rider B does not have to do anything and is riding legal.

So going back to the OP, if a 'lesser rider' is gunning it to try to pass you and then slow down, your options are to gun it even harder and force them into a penalty (i.e., don't let them get by you) or slow down after they get by to get out of the draft zone within 15 seconds. 

It may be 'bad form' to do this repeatedly (push past and then slow down), but it is perfectly legal.  It happens to me A LOT in longer races.  Many people ride up hills much harder than I do, but then ease up as they crest the hills.  I ride up the hills slower, but maintain my power over the top of the hills so begin accelerating much faster than they do.  If I get passed near the top of a hill, I almost always have to slow down my desired pace in order to drop back out of the zone first.  Yes, it's frustrating.  But my alternative is to ride harder than I'd like up the hills and cook my legs.  I prefer to save them for the run and pass those riders later.