Despite the thread starter being about as real as the voices in my head, this thread is now going to pop up in search results so I might as well dump something useful in it.
As Gunni said, Stellarium is a very nice astronomy app. It has now been
ported to N900 which makes it a realistic option for us.
For road navigation, Navit is already in the Angstrom repo. It uses the openstreetmap project for maps, which will please some people more than others. You will of course need USB/Bluetooth GPS receiver for this. Refer to the USB FAQ in my sig before buying USB gear.
Google Maps will not work on the Pandora like it does on the iPhone or in Android. You'll be able to use Google Maps via your web browser of course, but there will probably never be a dedicated app. I'm not sure what those dedicated apps offer to be honest, but if you're hoping for turn-by-turn navigation, don't expect it from Google.
There are plenty of possibilities for Pandora, but at the end of the day it depends on your needs. I'll be getting into all this stuff just for the fun of seeing what Pandora can do. But if you want polished commercial style navigation products along the lines of an iPhone app or a Navman, you won't get it here. Not yet anyway.