Default desktop clipboard manager
Are you aware that the default desktop already comes with a clipboard manager ? Admittely not quite as fancy as what you describe here.
It needs to "just work!"
The goal is to address what I believe is a major shortcoming in the current default, and to improve the user experience – especially for those coming from a Windows background.
As it was described to me
There's a clipboard saving protocol already there and support in gnome-settings-daemon. If parcellite "works" when that doesn't (like for firefox) than that indicates app bugs. If you save clipboard contents without using the protocol, then you are likely "stealing" the clipboard away from apps and that will cause massive problems with something like the GIMP that exports data in many formats - either you have to pick one format (causing a loss of flexibiliy), or you have to copy all the formats (massive memory and efficiency problem).
This would make this sound like a bad idea.
Don't see a problem
I just tested GIMP with and without the daemon running. GIMP behaved the same way, regardless of the daemon, whether the copy & paste action was performed while GIMP remained running or whether the copy & paste action was performed after reopening GIMP. I also watched the memory usage via System Monitor and it stayed stable under 1 MiB.
Parcellite somehow doesn't work with Inkscape Copy&Paste, just mentioning. I really really like this feature though, the current Clipboard state is simply not acceptable for a "modern desktop operating system".
Breaks copy & paste in Inkscape. It appears that this is a known issue with certain applications. Perhaps this feature could be held off until the problem is resolved. :-/
Update - Parcellite works
Parcellite 0.9.2 currently in Fedora 13 has no issues with Inkscape. We in Fusion Linux Fedora Remix have been using Parcellite as clipboard manager for last 2 releases with great success. Highly recommended.