So I recently made the switch to 64-bit Kubuntu. So far the experience has been relatively painless. Graphics card, audio, and peripheral drivers have all worked 100%. I hit one snag today attempting to install the Amazon MP3 Downloader. It threw an error message about requiring an i386 architecture.
I found a post by David Heffelfinger which walked me through setting up Amazon MP3 Downloader under 64-bit Ubuntu. The only issue it had was the link he provides for downloading the getlibs .deb package. I found a corrected location using this Ubuntu Forums topic.
I'm now happily 1-Clicking my paycheck away again :)