Settings -> Wireless & Networks -> Mobile Networks -> Data Enabled tick box, happened on a mate's Galaxy S too.
Tried this closet i could go was Settings -> Wireless & Networks -> Mobile Networks -> USE DATA PACKET tick box. This was unticked and still no 3 G or rather symbol for 3g appears sometime after turning on the phone but sign error for googlemail appears.
My wife now also has am gal S the t mobile sim does allow internet ( but not utube?) both on her phone and mine. My sim still can't access the internet on her phone
So potential problems
1) something wrong with my sim (but this is my second sim and the odds of both sims from different sources both shoing the same problem must be low).
2) Something wrong with the network, but I've seen my phone sitting on a desk with a ano. of other vodafone simmed phones all accessing a signal, I also travelled across several post codes from Essex to central London, all with no signal.
3) Problem with my googlemail account, but I'm able to log in via a PC and wifes mobile ?
4) Problem with the vodafone account itself.
This is doing my head in

Any suggestions. I suppose their is always factory reset but can#t see what that would achieve but finally elim possibility of phone itself causing the problem