I have got one of those radio transmitters for the iphone/touch and i was wondering if there was anywhere where i could get a list of empty frequencies or at least poorly populated ones!! I though 102.6 was free until tonight when something overrode it

Everything from 88 to 108 MHz is in use somewhere in the country, and there are pirate radio stations that won't appear on any list, so trial and error is your best bet.

Transmitters such as the Belkin Tunebase use Clear-Scan to find an unused frequency. Very useful, particularly if you are using the iPod/Phone for in-car entertainment.
Does yours not have something similar?

Yes it has the clearscan which quite happly finds a clear signal which more often than not last about a couple of miles. 102.6 has been the best for a while but fell over when i went over jawbone earlier. I know its so high up there it will pick up radio for miles.