It could be the forms API code ends up in an infinite loop over this little fella and, if so, i'm happy to fix it. Just need someone to reconfirm this happens (especially in another version of PHP).
I have had two instances of segfaults in PHP, though unrelated to FormsAPI. The first one I was able to track to an older version (Feb?) of the og_forum contrib module, in the function og_forum_db_rewrite_sql(). For some reason it was getting called again and again until PHP segfaulted. This happens with PHP 4.4. The second one was when I switched from Mandriva to Ubuntu, and from 4.3.x to 4.4. It would only happen when drupal was installed under a subdirectory, so I suspect a mod_rewrite bug, which someone reported on SuSE as well somewhere on Drupal.org.