I used to use Plain Clip for converting clipboard contents to plain text before pasting, but now I do it in one step with Quicksilver.
I created the following custom trigger:
Item: Clipbord Contents
Action: Paste as Plain Text
Shortcut: alt-v
Now if I do alt-v instead of cmd-v for pasting, it pastes as plain text.
Lorin's Tech Blog 
THANK YOU. This has been driving me crazy. I too used to use Plain Clip but it occurred to me there HAS to be a way to do this in Quicksilver. Most other tutorials I found were insanely complex, this made it so much easier.