V800 and o2 3G

[sidsnot]

I have an unlocked SE V800 on Vodafone. I also have a data enabled o2 SIM from work. The SIM works in the phone no problem but I was wondering if it will automaticaly switch to 3G if I am in an area covered by o2 3G ? o2 have no coverage where I am at so I cannot test.

Thanks

Alan