I would say imo the most important thing for the competition idea is to make it as absolutely visible and well known as possible, that'll keep up the interest. Perhaps start a new thread in a more mainstream area of the site, (as well as here) say, maemo.org > Talk > Devices > Nokia N900 (If you're specifically targeting N900 users) and request to have it stickied. I would imagine they would be ok with it considering it would be in the community's best interests as everyone would benefit.

Also, perhaps try and find out who to contact about the possibility of having a notice about the competition on the front page of the site, its updated with news and new apps, so why not community run competitions? Maybe whoever runs the site would be ok about having even just a small notice about it on the front page that stayed up for the duration you intend to have the competition running for. After all, it would only help stir up more interest!

GBAX are the guys behind most of the coding competitions on www.gp32x.com for open handheld consoles like the Gp32, Gp2x, Wiz, Dingoo and the imminent Pandora (which has very similar hardware to the N900) You'll notice they have a link to the competitions right from the front page of their site.

Here are GBAX's previous Coding competitions along with all the entries just to give you a feel for how they did it -

GBAX Coding Competitions

And heres this years competition, again with the entries, called the Pandora Angst coding competition

I hope some of this helps