Heather Leigh wrote an interesting blog about her opinion on this subject called IM Angst, quote
"What’s the deal with people you have never met before IMing you? This is analogous to interrupting a potentially important conversation (let’s not pretend all of my conversations are important but some of them are...some are even important AND interesting). In my opinion, IMing basically says, “I know you well enough to do this” (among friends) or “this is urgent” (among business associates). I cannot tell you how many times I receive IMs that fall into neither category. When a simple e-mail would suffice, IM is chosen for the immediacy (of the sender) without regard to the time of the receiver."
Suffice to say new habits and cultures will arise. Quickly, if appropriate, moving from e-mail to IM with Office Communicator has certainly lessened the amount of e-mails in my inbox and also moving from IM to phone or video/VoIP conferencing has shortened the amount of time in the IM space for my sake.