I first installed TruCrypt and created a 100MB file inside of Dropbox with AES-Serpent-Twofish encryption.
Then I mounted the file as a volume, then installed Pidgin Portable on the volume.
Everything seems to work fine, and is self contained. - persistent logging [in dropbox]
- encrypted logging [log files are inside the pidgin directory, inside the encrypted volume]
- the ability to turn off logging for a specific conversation [OTR pidgin/adium plugin]
- the ability to edit / delete parts of log files and have those changes eventually reflect to all other machines, Dropbox-style [can edit log files which are stored as html, if needed]
- multi-service (AIM, MSN, Yahoo, Gtalk, perhaps IRC too?) [yup]
Key management is the only issue, as I have never used TruCrypt before, I will leave that as an exercise for the OP.