Viral campaigns - how far 'off-brand' will companies go?

An interesting thought struck me the other day.

I got a viral mail with the subject line "So & so has sent you a BT Answer e-message". I actually opened it for a change and followed the link - to an animation that So & so had created for me. The thing that struck me was that, although the initial mail had made an open reference to BT, the actual site (www.psst.to) and animation were way off from the straight, conservative style you'd associate with BT - in terms of tone, design, etc. The messaging of the viral site, too, was not the hard sell you might expect from BT (in this case re their Answer 1571 thing). But it made me wonder how far companies are going with this sort of exercise...

What are other people finding? Are the big corporates finally shaking off their inherent caution and brand control freakery when tackling this supposedly brave new world of viral marketing? Are they creating viral applications that provide a separate, imaginative 'hook' to make you engage with it (rather than simply trying to flog you X or Y service)?

Or are you finding that they still view the medium with suspicion and shy away from creating anything remotely bold, preferring to stick with what their 'brand guardians' and 50-yr old marketing managers tell them instead?

It would be interesting to hear both of examples that you've received and ones that you've worked on.

Thanks

Sam