A very good GPS review site is http://www.gpspassion.com
... I visit there from time to time.
It's getting cheaper and cheaper, you can get GPS for well under $100 for laptops now, and BlueTooth GPS units for under $150. The wireless Bluetooth GPS units are the size of a pack of cards.
These might be usable with the rumored Bluetooth-capable Blackberry 7290 too if there's a Bluetooth serial profile for the 7290, then someone could write a GPS navigation app for the 7290, although the 32MB of RAM would only be enough to hold navigation maps for one major city or a few minor cities.
Then the question remains about whether the CPU of the Blackberry is fast enough for map rendering (Probably yes, at a slow update rate). The Blackberry screen at only 240x240 for the bigger models (77XX) and 240x160 for the smaller models (72XX) may be somewhat cramped, but superior to older PalmOS handhelds at 160x160. It would be useable for finding your way around an unfamiliar city, and you'd just keep your Bluetooth GPS unit inside your briefcase or backpack etc. (many newer GPS based on newer GPS chipsets are now sensitive enough to actually work indoors in non-metal-frame buildings or stowed away without needing line-of-sight to GPS satellites. Some even have accelerometer dead-reckoning for GPS through tunnels and urban canyons with intermittent satellite view) As a result, GPS is a lot more discreet than they used to because you no longer need to pull out an antenna anymore, packed away in a pocket, briefcase, backpack, or glovebox works well enough for the better Bluetooth GPS units for most situations. And you can reuse the same Bluetooth GPS unit with your laptop, with your desktop, with your PDA, and maybe with the Blackberry 7290 too.