Windows Mobile 7?
Recent article in lowyat.net on WM7 seems very dissapointing.

Unlike other smartphones, the N900 is a very powerful device. While others are more of a phone (phone/mobile based OS e.g. Windows for Mobile VS Windows for PC), the N900 is an internet tablet with phone functionality. Imagine a tablet pc (or notebook with touchscreen) with the size of a handphone, and it comes with phone functionality.

Anyway, talk about waiting for Windows Mobile 7. Why not check out this smartphone ITG XpPhone which comes with Windows XP! Say goodbye to mobile based browser, mobile this, mobile that, etc. It's Windows XP running on the mobile phone! But the looks of the phone isn't very attractive tough and it's HUGE.
http://www.xpphone.com/en/product/configuration.html

Now that's a true contender for the N900; both are not your usual smartphone.

The default is resolution is 3.5M resolution (widescreen).

The camera function is pretty basic, probably that's one reason; and ask your customer to choose lower ISO settings for less noise but will not be clear in low light conditions. In Automatic mode, the camera will auto-adjust to higher ISO settings for brighter picture resulting to more noise as I've read in some reviews that the N900 image noise is pretty high.

It's not the best 5MP camera phone, but I think it's OK. Would be better if there're more settings/features available. For N95 comparison, I'm not sure, maybe try ask your customers to play with the camera settings on the N900 first and compare the results.

Depends what are you looking for in the phone. X10 and N900 is very different in terms of design.

X10 lacks of qwerty keyboard, but it has much bigger display, 8MP cam, 1Ghz snapdragon processor, slimmer, plus it's a SONY.
It runs on Android OS, which have very good support, fast updates and also apps/games support/availability.
If you're looking for good cam, good app/games support and doesn't mind the qwerty keyboard, then X10 is your answer?
If you want qwerty keyboard however, you still can consider Motorola Milestone (which look a lot like the N900 but with slighty bigger display yet it's much slimmer).
Both phone lacks of internal storage/memory and requires micro SD card.

N900 has 32GB internal storage (2GB reserved + 768 for Virtual Memory), and you can add micro SD up to 32GB!
It runs on Maemo5 OS, VERY open linux based OS, the possibilities are basically limitless and you have root access, you can do so much with this device. But because only N900 uses this OS and currently not much user... there aren't so much community support and also not so much app/games provided by those well known companies. The future is still uncertain for this, possibly it will BLOOM and become very great or possibly it'll only get most support from the Maemo community. Due to it's capabilities, many things are possible on the N900, they've done so much with this N900..... it's just launch in December 2009 and only in US and EU, but yet it's gaining lots of popularity.
Nonetheless, if you're looking for "open" OS or little or no restrictions, this is the device MUST get. Plus, you should check out N900's multitasking capabilities, in YouTube someone actually open up > 15 apps running.

By the way, in terms of OS design or user interface, you can check out both too as they're very different too. They both look very different. Maemo5 have more futuristic look (imo), nice progress bars, nice Media Player home, plus, you should really check out the Desktop (some call it Workspace) which is really unique. Then again, some prefer Android's OS design, very "robotic" feel?