I am very close to taking the plunge and buying the N900 because it seems to be the best "hacker"/"open source" phone out there (better than all the Android phones with the locked bootloaders where the manufacturers fight you at every step of the way)

Some questions about the phone (based on whatever the latest official software version is):
1.Does the N900 use any binary kernel modules or is all the secret stuff in userspace?

2.Can I program this thing in QT and if so, how different is programming this in QT vs programming say desktop Ubuntu in QT?

3.Does this phone support 3D graphics? (and if so what version(s)
of OpenGL are supported)

4.If it does support 3D graphics, how is the support implemented? Binary kernel blob? Binary userspace X11 driver? Open source driver? Software OpenGL stack with no GPU involvement? Some combination of the above?

5.Does the N900 include maps and if so is there a list of the features the N900 maps app supports? (i.e. is there a way to get directions/turn-by-turn here in Australia for free either officially or otherwise or would I have to pay for it?)

Yes, the latest upgrade (PR 1.3) brought QT 4.7. Non-QT interfaces from Ubuntu may or may not be available.
The SGX 530 GPU provides support for OpenGL 2.0
The GPU is definitely involved, as the CPU would not be nearly beefy enough to provide software rendering. I'm not sure how open the driver is.
There's a free map app called Mappero which can uses Google for routing and can then provide turn-by turn directions.

Hm... I know the kernel is (more or less) open - there are a number of custom kernels floating around.

As far as I know, the GPU driver is -closed-.

QT wise, I haven't done any "true" QT work, but I did some work with PyQt4 - I was able to take an application written on my Debian desktop and run it without any changes on my n900. Admittedly, it looked quite a bit different, but it ran without errors.

Is that OpenGL or OpenGL ES?

OpenGL ES.

As mentioned previously Qt is very well supported in Meamo5 (all systems have quirks of course) and it's not wildly different to a desktop environment - there are a few bits to pick up on your travels - so if your comfortable with Qt you should be fine with it. As Meego has Qt at it's core as well it's a good route to go development wise.

But you aren't just limited to Qt with c++, python support is good and well used, Gtk is accessible with c, c++, python and vala too. It's a fun device to work with, not too easy that there's no challenge, not too hard to completely put you off.