Sadly, I can't recommend the Dash Genie (above) for the N900

With the N900 in portrait, the protruding camera cover prevents the phone lying flat against the holder and as a result it only manages to make about 25% contact with the adhesive 'blob'. This does hold the phone, but not strongly. A solution (which I'll try) would be to cut the holder off above the 'blob' which should then allow the N900 to lie flat against the 'blob and make full contact.

With the N900 in landscape, again the protruding camera lens prevents a full contact, but you can offset the phone to the left to avoid this. Even offset the phone only makes about 75% contact, as the 'blob' is slightly too high. You could physically raise the phone ~15mm by placing something underneath it to make fuller contact with the adhesive blob.

Finally, I don't think the matt finish and the engraved "NOKIA NSeries" branding on the back of the N900 lends itself to making a good adhesion.

Conclusion: the holder holds the N900 but not securely. I'd therefore say that this holder is not suitable for the N900