I'm thinking about replacing an iPhone with Android device, but before I do that I'd like to know which versions of Android support multiple Exchange accounts? Are all main features (Mail/Calendar/Contacts) supported when several accounts are set up?
From Android 2.0 Exchange support was added and this included multiple exchange accounts. http://developer.android.com/sdk/android-2.0-highlights.html
The highlights there mention "Handset manufacturers can choose whether to include Exchange support in their devices." But all the the higher end devises (Galaxy S, Nexus One, HTC Desire etc) will most certainly support it.
Yes, at least on 2.1+. I do have the Samung TouchWiz UI though, and the email app is subtly different. I would expect it to be the same in this regard however. Note that the Email app is terrible, though, and may not otherwise behave how you want. I recommend a third-party app or forwarding your email to Gmail.