My 3 Skypephone S2 at some point started to run out of battery after being on for about 20 hours or less. And this is not because a Java program crashed, which can be a reason, too. It was after a full charge over night and switching it off and on. Eventually it turned out the problem was that I had still activated the USB Modem setting in Connections. Once I turned it back to blue tooth mode and turned off bluetooth after that the problem was gone and the battery lasted a *lot* longer again. Anyways, the battery delivered with the phone seems to be not very good in any case, so you might want to get a new one. The big question is just: where?
Update: It works! The only problem is you need to leave out the battery at least over night afterwards.
If you like this, why not order a Three UK Sim with my agent link (or send the link to friends). You get an extra 5 quid or so AFAIK.