Multiple BIS registrations on one Phone

I've got a 8120 on a Vodafone contract from work. It's got BIS (and the remains of a BES account that we dumped a while ago) on it which sorts out my email etc...

What happens if I want to go on holiday and put my Orange "Holiday SIM" in it?
Well I'll tell you, nothing, no signal.
So what happens if I go onto the Orange BIS website and register my 8120's PIN and IMEI number with them. Will they take over from Vodafone and it'll knacker my phone up or will it work nicely for 2 weeks and then I can go back to Voda when I get back from Spain ?
Or can you register with 2 different BIS providers at the same time and swap the SIMs round "willy nilly" ?

anyone got any good ideas ?