As a new owner of a N900 I have to say that offline GPS is useless for the following reason:

Without a data plan or wireless connection OVI Maps will constantly (every 5 seconds or so) show an errormessage blocking the view to the map:

"No saved connections available"

And this even if it has GPS lock and shows your correct location.
This is very annoying - specially while driving!

Already disabled things like:
Settings - Network positioning
Internet Connections - Switch to Wi-Fi when available

and also disabled Wi-Fi with the program called Wi-Fi switcher but to no avail.

Why does OVI maps not have a possibility to disregard assisted GPS?

Offline GPS is the only GPS mode I will use, so if I don't find a way to switch this annoying error message off I will return this device and get my money back.

u can install i2c-network-dummy or something like this
creates a dummy network connection

yo dude, the way i got around this was to put the phone in offline mode using the cellula modem controll buttons (i like in extras-devel so be warned it may not be safe etc) then open ovi maps, and hopefully should be all good! down side to this is u get no text msgs or phone calls while using it in offline mode!

I think the GPS reciever is a very weak one that is placed into all devices for the purpose of cost and space.
Even if you got it working (without the issues), the GPS probably wont lock and be quite useless. Remember the N95?
Thats why these are called aGPS.
A true GPS can do the work without assistance, such as the SIRF Star III chip. Have a look into the Mitac Mio (Digiwalker) A702, a beast of a smartphone in 2007.

A-GPS only means that the chip uses the network to download ephemeris, and therefore obtain a fix in 10-20 seconds (by getting it from satellit it would take longer). After the chip has gotten a fix, there is no difference between a GPS and an A-GPS one.

Just a fyi: The n900 has a true gps chip -- I don't have a sim card (data or phone plan) and I frequently use Maemo Mapper on it w/o any data connection around for it to use to even get a gps fix. You can watch in Maemo Mapper how many sats it has found if you are curious and how strong the signal is.

I've have no issues with Maemo Mapper; I don't use the built in Ovi Map as I found Maemo Mapper easier to use. So I haven't looked at Ovi Maps in ages.

The only down side to MM is you have to download the tiles; which is a pain. But once you have them it works really well for what I need.

Nathan.