Touchdown (for Android) http://www.nitrodesk.com/dk_touchdownFeatures.aspx keeps all of the Exchange Activesync'd data separate from the main email on the phone, so when you get wiped only corporate email/calendar get wiped. I don't think anything similar exists for iPhone.

I can't speak for iPhone (though I imagine it's similar), but pretty much all your data can be recovered to your phone just by logging into your Google Account, from what I understand. If you use AppBrain, the apps you have installed will be saved too.

You would need a lot of granularity - wipe business emails but not personal ones, wipe documents that were mentioned your employer, but not your CV, business calls/contacts but not personal ones to workmates?

The question is whether an Exchange wipe should wipe stuff that isn't managed by Exchange (mail / calendar / contacts). That seems like a reasonable level of granularity.

OTOH, a corporation really does want to wipe the whole thing if it's lost, which is why the Exchange level wipe works this way. It's listed as a business feature: http://www.apple.com/iphone/business/integration/