No change log seems to be available but you can get upgraded via app world. This is the first update that IM clients have received in almost 6 months, so if you notice anything new leave a comment and let us know what you find.
Instant Messaging Clients have been updated to version 2.5.125. I use MSN and Gtalk and I looked at Yahoo Messenger so I know those have received the update, The latest update should be available via your BlackBerry App World, so you can download via BlackBerry App World. If the update does not show for you in app world you can wait or try holding ALT while hitting R S and T.