Well, it's all done and dusted. I submitted in June, had the viva on Friday and recovered from the hangover (although now I'm ill - Sod's Law). I thought I'd share my experiences with everyone, since hopefully people will find it useful considering that a lot are horror stories. For background, I'm a largely self-funded (fee-scholarship and several jobs) Politics PhD who started in Sept 2009.
My prep for the viva consisted of reading the thesis once or twice and looking at a list of questions my sups and friends had sent me - a couple actually came up. The main thing I did, though, that stopped me freaking out was tell myself that actually, I'd been prepping for this exam for four years.
Anyhow, I went in suited and booted. The first thing they said was "We both enjoyed the thesis, so don't worry", which is always a good start, and then "You can take your tie or jacket off if it makes you more comfortable" (I didn't, if I'm going to the trouble of wearing it, damnit I'm wearing it). In total it took about 1 & 1/2 hours, with a wide range of questions being brought up and various decisions being questioned (they completely trashed my decision to have a two page conclusion, but accepted the reasoning). Anyhow, after the various questions about how I came to do it, what I thought the argument was, etc, we spent about 30 mins talking about publications and how to "butcher the carcass" for articles. Then they sent me out for a minute to confer, then called me back, congratulated me and said the thesis needed no corrections.
Frankly, it was the nicest examination I've ever done. The best advice I got (in retrospect) was enjoy it and remember, it's your thesis.