Just fired up a Nokia LD-3W BT GPS unit that I got with an old Nokia tablet. The X10 will see it fine, but any attempt to use Google Maps or ALK CoPilot turns the phone's internal GPS on automatically. So based on that quick test I'd say that an external GPS is a non-starter. Although - according to the X10, the Nokia unit I'm trying to use is actually coming up with the wireless headset icon, so maybe it's merely a question of drivers - maybe the X10's are set to only recognise certain categories of BT device.
If you've got a decent phone shop near you that you're going to buy the X10 from, you could always take along your GPS unit and explain what you're looking for. If they're good folks then they'll let you spend a couple of minutes with a demo phone to check connectivity. It shouldn't take long. 
By the way, I've always found the X10's GPS to actually be quite good - certainly light years better than the old (SirfStar II-based) one in the Nokia N95 phone I had previously. And it actually seems to get a cold fix quicker than the (SirfStar III-based) Nokia LD-3W. Accuracy is pretty passable, certainly good enough to use for car satnav (which is why I've got the CoPilot application). Only downside - for me at least - is a tendency to "wander" if the unit's sitting still - keeping it moving seems to hold the accuracy, don't ask me to explain why. 
So I guess what I'm saying is that you might find that the internal GPS receiver is 'good enough' for what you want - Sony seems to have been blessed and missed the GPS flaws that appear to be plaguing some of the HTC phones and the Samsung Galaxy S.
Hope this helps, Bob.
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