Skype and gTalk protocols are pretty wasteful when it comes to communication with their servers, Skype esspecially - pings and status requests are very frequent and have quite large footprint (just two accounts will cost you approx 1MB of data / hr) - so they are constantly utilizing the connection, and that power has to come from somewhere. At least Skype promised to trim their protocol (and not using your connection as a gateway as they actually do from time to time which can eat tens of MB per hour) for mobile usage, since more and more phones are coming with VoIP abilities. As for the gTalk, since it's essentially a Jabber protocol, optimizations can be made but there is no much room for improvement.

Still, there is no pocketable device out there that can keep real, non-relayed connections to various IM and VoIP services (not to mention the integration), longer than the N900. If you are not happy, your expectations are way too larger than the current commercialy available technology can provide.