The osx-pl2303 project proposes a Mac OS X driver for the PL2303 Serial/USB convertor. Unfortunately, the current version do not support 64bits kernel.

Here's a patch to allow to compile the source code for the x86_64 architecture (Intel 64bits):

# svn checkout svn:// osx-pl2303
# cd osx-pl2303
# ftp
# patch < osx-pl2303-intel64.patch

And an installation package (the original PPC and Intel 32bits versions, with additional Intel 64bits support). Tested on Mac OS X Lion (10.7.x).