I know each user has different needs but YIKES would that be one giant inbox for me if all of my sms, mms, e-mail, IM's, etc. flooded into it. Even on my BB, I make sure to filter my unified inbox to just my e-mail accounts.
But to answer the question, I haven't seen any app that does consolidation of all of that. Just ones for e-mail specifically or IM specifically.