Just spent many minutes on a wild goose chase and the underlying cause was that I had a <div> and a <textarea> that shared the same id. $(id) returned the <div>, most likely because it occured first in the document. I then got an error because <div> doesn't response to activate() ... that's a Prototype method attached to form elements.
I would be tempted to make Tapestry enforce this (that the id attribute for any node was unique), but that won't proof it against partial renders under Ajax, so I don't know that there's any point.
What's really going on is that CSS rules based on element id rather than element class are evil, and should only be used as a last resort. The free CSS template I got off the web uses too many explicit ids where it could be using CSS classes. You get what you pay for.