I'd love to see an AIM/MSN/Yahoo IM client that uses GPRS/EDGE/3G instead of our SMS message bucket. Having to pony up and additional $10/mo ($120/year) for unlimited texts kinda sucks.
What would make it even better is to have a component that would allow you to provide your own backend proxy services. Essentially, your computer at home would connect to AIM/whatever and your G1 would connect to your computer. That way you always show up as being online, even if your G1 drops into a no coverage area. While you are not connected, your home computer could queue up messages that would get delivered when you get back into a service area. Conversely, you'd be able to "send" messages when you are in a no coverage area, they'd be queued up and sent out when you got back into a coverage area.
In other words, extremely similar to how the sidekick works, it's just that you provide the backend server, rather than tmobile.