Mine has been pampered, and stays in a Nutshell leather case when I'm not using it, but while in Santa Clara one of the more marketing type Nokia guys had a habit of dropping/throwing his n900 down to the table whenever he was done doing something with it.

Somewhat unsettling, but did not notice any issues/scratches resulting from this form of a stress test/demonstration, not that I'd recommend trying it at home however.

From the disassembly video's I've seen the device seems quite sturdy aside from two potential weak spots:

Kickstand feels very flimsy, espescially when compared to the stability/utility of the 810's kickstand (I really like sliding the 810's kickstand over my hand, and the tablet just sits there nice and comfy).

The usb charger port, only because it's so easy to be snagged from a cable incident. From what I understand though, an increase in durability/longevity is the primary motivation behind using usb micro vs the more common usb mini form factor.