Nobody seems to have mentioned using Compact Flash GPS units, here or elsewhere AFAIK, perhaps because people generally assume (as I did) they don't work with the nB/S7.
However, this almost certainly isn't the case.

Recent versions of Route & Street Planner Millennium (& CityMaps) have an undocumented facility to change the port via which GPS data is received:
Menu > Map > Preferences > GPS tab > Settings > Comm::0 is the normal, default setting for the RS232 serial port. Setting this to Comm::1 should set data to be received from the PCMCIA slot!
The Help file makes no mention of this, just referring to standard serial port connection. I couldn't see anything at TomTom's website, either.
All credit to Chris Handley for discovering this & pointing it out to me. 
His upcoming MovingMaps app (looking very nice in Beta now, Chris!) will support PCMCIA input.
Unfortunately, this means I now just *have* to get a CF GPS - although I'm concerned about reception/shielding issues for in-car use (do any CF units come with, or accept, extension antennae?), it would be such a neat & efficient (single-handed) solution for other 'clear view' handheld uses.
I don't know anyone who has a CF GPS unit to check this out for sure (and do take a precautionary approach if getting one and relying on this info) - can anyone help? Be nice to have this confirmed for certain. All I know is when I select Comm::1 in Street Planner and then select Comm::0 again, I can hear my Gold Card click through the speaker, just like it does when I disconnect from the internet!... I suppose if I could find a website which simulated a GPS output?...
eXpansys offer a Pretec CF unit at £160, which comes bundled with CityMaps as well. It's current consumption is very low (almost half a low power WiFi card apparently):
Maximum: 160mA
Typical: 80mA
Trickle: 35mA
My existing Garmin III eats AAs at an alarming speed, four at a time! BTW, Alfaloco, apart from this, it works great with my nB/5mx.
Anyone know of other brands/models/sources/prices/specs?
I think Comm port selection (and GPS 'calibration') were added to Route/Street Millennium in the last few versions, but any copy of the Millennium series can be upgraded free to Street v2.1 & Route v2.13 - these are the most up to date versions prior to Route/CityMaps v3.x.
Cheers, Lewis