If you have a Prolific 2303 based USB to serial adapter and it no longer works under Lion, installing the Snow Leopard 64-bit driver may fix the issue.
Download the driver from Prolific's site here.
[crarko adds: I post this as a generic hint; if a device breaks under Lion check with the vendor to see if 64-bit drivers are available for it.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110721092446939