If you put 'sasl' in USE flags, sendmail can't compile because it can't find sasl.h (which is in /usr/include/sasl).
Created attachment 2226 [details, diff] Patch for sendmail ebuild to make sasl work. cvs diff -u patch made in /usr/portage
Hmmm. I think that patch clobbers the user's CFLAGS. I've fixed that problem and there's now a new ebuild on CVS. Thanks!