It just got easier. Here's my favorite Web 2.0 freebie app of the week: DexRex.
DexRex allows you to archive all your old chat threads. Here's a list of all the IM and network clients it supports, so far.
Your web-based archive stored wtih DexRex is searchable; meaning you can find that phone number of a contact buried in an IM from last week or reconstruct a conversation on a project with a specific colleague.
This is a handly little tool for smaller businesses and the self-employed crowd. If your company has special compliance issues, I'd take a pass for now.