That is interesting (to me, at least).
Yes I have been asked to join startup teams for equity, and I have
done a couple because they were cool projects. This includes Usable
before we got on the Ignite100. Subject to funding being signed off I
am about to do it again later this year. Yes, as Robin hinted at, I
was asked because of reputation and because they know they can not pay
a going rate for the work.
All that being said I can't say I have ever been approached as a code
monkey or web developer in this way, or seen it mentioned in a job
advert for that matter. Perhaps I have just been lucky or that I am
not associated with kind of work that people expect for free.
Alistair