If I run IM app (Numbuzz, Slick and etc) on N95 or 5800XM, the battery will be gone in hours. I don't think it's a bug in specific to N900, it's just how it is if running IM on any device, probably the protocol was designed for always connected desktop, can also be protocol specific. I don't think it's Nokia or the firmware to be blamed. There might be some tricks can be applied to use power more efficiently, which would be up to the developer who made the software.