Going from 2007 HMC4.5 to 2010 SP1/Hosted was painful. However we got it done thinking it will be the last time we have to do a cross-forest migration.
Now comes 2010 SP2, and here we go again.
Having separate environments would never work, because ultimately you will have to move everyone over to the new system (why else are clients paying for hosting and SPLA, if not to be on the newest system).
I am shocked and disappointed with how Microsoft has been treating Partners with the hosted platform. Firstly the SPLA pricing is not line with Office365, secondly we have our hand tied behind our back as we cannot offer OfficePlus for local use (but Office365 can), thirdly MS has provided partners nothing but false promises about the platform going forward and getting a straight answer from MS about the future plans is simply no possible.
Maybe GoogleApps is the way to go.