There are many more differences between Symbian and Linux besides the openness of the source code.

Depending on level of support I'd say lack of official backports for:
1) Security and reliability fixes.
2) Features.

There are many shades of grey between those. What is IE7, 1 or 2? A-GPS, 1 or 2?

I imagine a N810WE customer would feel bad knowing there is a N900HSPA, while she'd like to have bought a N900WE or N900HSPA with Maemo 5. It'd depend on many factors though such as but not limited to price as well as the timeline between the releases. IOW, releasing the N810WE before the announcements wih details about N900 and Maemo 5 would have been smarter, business-wise.

Consider I bought a N95 in july. In august I hear about the N96 release. Now, once the N96 is released, the N95 will drop in price because of a new product in this series. This is another reason why a customer could feel bad (price example). However, the good news is that many new features for N96 (software-wise) have (or will) see light in the various N95 versions.

OTOH, new doesn't mean better, and when Maemo 5 comes out there will be rough edges, too. Also, a community supported OS2008HE might cut it for some users.