I was able to remove the trim from my 2007 by just slipping a thin blade common screwdriver under the edge and prying upward. You need to be careful not to distort or gouge the trim while prying on it. There are fasteners on the back of the trim that fit in holes in the dash. After that the various dash components remove by using a nutdriver or smallsocket on the hex headed screws.
BTW,is this a new replacement radio with XM or an aftermarket addon? My 07 didn't come with XM either and while I have a visor mount Sirius, I might like to geta dash mount XM. XM is currently offering prior customers half off the annual subscription of $150, which is much cheaper than Sirius offer of $110. They will be merging in 08 anyway.