Alternatives to maemo exist.
1. Mer
2. Debian
3. Archmobile
4. Other distros (?)

There are also some community efforts to create a Qt alternative to Hildon. For example the team i'm part of is doing this software http://gitorious.org/opendesk

Cheers

To me it seems like it really just depends on who you are, where you're coming form, and what your needs are.

If you want the latest greatest coolest gadget, then yes, you could easily end up waiting forever and/or be disappointed by how quickly any device becomes obsolete. If the N900 serves your purposes best of the devices currently available and you need something to carry out functions you can't currently do on whatever device you have, then it's probably a good choice; that is, you shouldn't get the N900 to fill future needs, you should get it because it fills current ones. If you're a user of the N810 and other previous devices, as BatPenguin discusses, then maybe the N900 isn't enough of a leap forward to make it worthwhile.

Once thing I wonder in addtion is who's to say that Maemo 6 will really be the end point? Nokia isn't going to just stop developing. Might not there be a Maemo 7? 8? 9?

All that said, I think it is legitimate both to get the N900 now for what it is and to be concerned about whether a platform is really mature yet. From what I've read, Maemo 5 sounds pretty stable and not especially buggy. But still, the difference between versions isn't always just about the newest features, it can also be about reaching a level of maturity and usability. Hello Windows 2000/XP vs. everything that came before it.

I look forward to the day when you can just install a distro of choice on your phone, although I realize this may be a more complicated endeavor with mobile devices than with computers. But that aside, will any of these actually run on the N900 as the base OS itself (i.e. not in a window like Easy Debian)? And still give you all the features you would want, like the phone working, etc.? (By the way, I love Arch.)

Archmobile already runs on N810, but not all functionality is present. You can check success stories on our wiki. N900 as i saw is more open than n810 and that would allow even 3D acceleration (you'll have to download binary from nokia to make it work)

There are two awesome projects that we are following for phone functionality: FSO (Started by Openmoko) and Ofono (by Nokia and Intel). They are both good middleware for getting phone apps done easily. In fact UIs in both cases should only make dbus calls to make a phone call and receive dbus signals to receive calls.

Qt and Qt/embedded have support for dbus, so the job can be done easily.

Maybe the origin is a myth, but the concept is extremely real. It's what microsoft used to kill off competitors like GEM in the early days, by announcing that in a few months they'll churn out something bigger, better and smoother. Which of course they didn't. This is called FUD (fear, uncertainty, doubt) and it sounds like a step-brother of this osbourne effect.
Naturally, nokia isn't going to use unfair (but unfortunately legal) competition tactics against itself unless they're utter *****s. Which they just might not be.