Hi Charlie,
The themes and layouts should not be affected as (at least the ones I have looked through) they were created with web standards in mind.
As far as the extensions go, even though that have not yet been updates yet specifically for CS5, I can say that the majority of the extensions and features actually work now in CS5. I have only found a few minor issues with using the current extensions in CS5. So if you did get them now and installed them, I think you would get use out of them currently even before the updates are released.
Regards