thanks for the reply. i have figured out a sort of workaround, though i still cannot figure out the source of the problem. my two goals for this mod_rewrite were to 1) remove the .php from the end of the url, and 2) shorten really long urls to something short and easy to remember:
root/folder01/folder02/folder03/file.php => root/nickname (without extension)
it is not a redirect, just telling apache to look for "root/nickname" and translating it back to "root/folder01/folder02/folder03/file.php" or whatever. that's the way i understand it anyway, but i could be wrong.
anyway, it seems that something in "signup/?$ /signup.php" or the actual code in signup.php itself was causing an error. i'm still not sure what though. in the course of trying different things, i discovered that other nicknames worked fine - "signup/" in particular seemed to cause the most grief.
so i have mapped out all of my directory structure now, and all that the user sees are short, catchy urls.