FWIW, if you come to this thread feeling the pain -- here's a solution that DID work for me.
If you are on 10.5.2 or later, you can't run Beta 58 -- the social bookmarking doesn't work. Go back to Beta 54. To prove it -- I uninstalled absolutely EVERYTHING quicksilver and then re-installed b58 and had the same problems. I removed preferences, manual files, everything. I then uninstalled everything AGAIN, and installed b54 and the problem was solved. When looking at the console while trying to manually update the bookmarks list in the catalog, I got the message "Couldn't find password. URL :del.icio.us (null) http" over and over. My wager is that the plugin didn't store the password correctly for 10.5.x OS references under the b58 version.
