This is actually sort of a mystery. I had the same issue on my iPad a while back. Some say it's data that syncs with your calendar, contacts, etc. and others say its history, cookies, and cache data from Safari. SMS/MMS message I think also go into "other." You might try clearing all this and then restart your phone. In the end, it could be a number of things, which is why it's a mystery.
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clearing up old text messages dropped me from 1.9 gigs to 1.06. almost a whole gig! the history and cookies and stuff dropped it like .05 gigs not worth it
I had this problem at one point on my iPad. I believe one of my apps - a pdf reader - was failing to clear app-optimized rendering of pages of .pdf files. I stopped using the app and have not had a problem since. I have heard that there are other apps that spew junk data like this.
There are, however, some legitimate things that are categorized as "Other" - MMS and SMS as mentioned earlier, cover art from music files (you may have some that are ridiculously large res), and temporary internet files & cached data. You can purge caches from apps in the "Settings" for most apps.
While I was using the app, I found that I could clear the "other" data by using the Restore function. It took a lot of time to restore the iPad, but afterwards the "Other" data (which at one point took up more than 4GB) would be a much more reasonable .3GB.