From my experience with a Pearl and now an 8330, the gps on BB's just plain sucks!
My Pearl never got a GPS satelite. My Curve will lock onto a satelite but it is questionable at best. If you really want to use your bb for navigation you would be better off to buy a gps "puck" and pair it with your bb via bluetooth. You might also try deleting your cache and downloaded data. Sometimes that will work for me when I'm having problems getting a lock.
Posted from my CrackBerry at wapforums.crackberry.com