checkout the ebay link here for a N900 specific holder

The N900 specific part of this should be able to replace the N8 specific part in this ebay item http://cgi.ebay.co.uk/Bike-Bicycle-M...item53e5749d5c

If you look at the photos of this item which is a universal phone holer (which I found with the search terms N900 bicycle) http://cgi.ebay.co.uk/Bicycle-Bike-M...item2c59d5a701 you can see the 4 clip/posts which attach the phone part of the unit to the handlebar mount. The bicycle mounting part is actually exactly the same as the one for the 5230 I bought.

Make sure you have the thing strapped down tight though cos my 5230 flew out of a holder when I jumped a kerb at about 25mph. The build quality of Nokia ensured that the phone was fine though, although I's not want to try it with my N900, which now stays in a pocket on my racktop bag when cycling.

I also find this helps GPS and 3G signal quality slightly.