This is pretty old news now since it was news around the time I left for San Francisco, but I think it's humorous enough that it's still worth mentioning.
People ask all the time about connecting to the "Big 4" networks, like AIM or MSN. Our reasons for not doing this have always been the same since day 1. One of those reasons is that your software becomes reliant on factors outside of your control, and that's exactly what has led to a recent problem with every 3rd party client that connects to MSN.
As reported at CRN, the recent Internet Explorer 7 Beta breaks MSN connectivity using 3rd party clients, including Trillian or GAIM. It actually looks like it causes Trillian to outright crash unless you remove the new msn.dll. Despite the CRN article indicating that Cerulean wasn't planning to release an update (I'm sure that made people really happy!), a new dll is available from Cerulean.
It's also a sharp reminder of how easy it is to break 3rd party access, even if it can be easily circumvented. Does anyone else remember downloading patches almost daily for connecting to AIM a few years ago? What a pain. We still have some users connecting flawlessly with versions of BitWise that came out 18 months ago... no patches required.