You'll find it on the market already, from various providers including T-Mobile (MDA Compact III), and O2 (XDA Orbit), at subsidised prices. Alternatively you can purchase a sim-free HTC-branded P3300 from multple providers in the UK.

A quick examination of the specs of the HTC P3300 present a reasonable indication of the capabilities of the smartphone.
O2 and T-mobile provide their own GPS routing software, but the HTC branded phones come with TomTom navigator 6. A 'Taster' pack is available with one free city map, or a Premium pack is available with the complete set of Western European maps.
The device also comes with installable GPRS-data tracking software, so you can keep an eye on your data spend, when not using the in-built WiFi. Note that the T-Mobile version doesn't come with WiFi, which is a curious omission by the company.
The usual Windows Mobile fare is included, but the device has yet to be updated to the newly announced Windows Mobile 6, albeit there seems to be little difference between the two mobile OS implementations.
The package come with headphone to connect to the USB port of the device, but a headphone jack would of have been a nice inclusion.
Also to note is the annoying placement of the micro SD slot - underneath the SIM which itself is under the battery. It's about time manufacturers stopped doing this, and utilised easily accessible external storage slots.
Though the device is very small, more standard (and available in larger capacities) mini SD would have been a better fit.
The 'HTC RollR' which is essentially a mini trackball and track wheel is fitted to the bottom of the device. This rotating wheel and trackball combo is effective and very easy to use, and adds much to the device's appeal.
The 2 megapixel camera takes fairly decent snaps, and is easily as good as most mobile phone built-in cameras.
The screen is just large enough to be effectively used, and is pretty good in direct sunlight - a common problem with mobile devices.
We found call quality to be on par with a normal mobile, neither brilliant nor terrible.
A few more pics now for your viewing pleasure. First up is another front view so you can see how thin the device is. It's probably about as thin as most standard mobile phones, if not thinner.

A view from the rear.
Side on view, note the power button at the top right and the camera button at the bottom left.
In Short
A fantastic smartphone, with the full capabilities of a Windows Mobile 5 PDA, but with full telephony support as
well as built-in GPS.
The navigation pad with its small track-ball and optional mouse-like behaviour are a real boon, and if you're fed up with the stylus they're comfortable alternatives.
We can't help but recommend this to anyone who's looking for a new smartphone. You won't find a smaller compact PDA-phone combo with GPS in the market, and a plethora of accessories are already available for this well established device.
It's probable this phone would get 9/10, pushing 10/10, if it was 3G capable, but the small-GPS-smartphone with HSDPA is still elusive, this is as good as it gets - for now.
A note to HTC. In the next iteration of this device, please give us our 3G, a headphone jack and easily removable storage, but don't remove the GPS! A slightly faster CPU wouldn't go a miss too.
Despite the niggles, if you're after a new phone with GPS built-in and can't wait for the Nokia N95, or prefer a Windows Mobile device, go get one now, you won't be disappointed - this hack is now using the P3300 as his main mobile. µ
The Good
In-built GPS and mapping software.
Windows Mobile has a lot of software support.
Nice, compact PDA and mobile all-in-one.
The Bad
200MHz CPU - can choke at times.
Limited memory - micro SD needs to up its capacity.
SD slot is under battery.
No headphone jack.
The Ugly
No 3G support.
Bartender's Report

See also
Nokia to offer free mapping software
Nokia N800 examined up-close
Nokia is winning the price war
Nokia reveals N-Gage plans