The Fremantle SDK with the nokia-binaries repo has the fonts available under the package name ui-fonts, IIRC, which you can then copy to your computer's fonts folder.
Can I download Nokia Sans font anywhere to use for UI mockups ? TrueType or OpenType..? Suppose it is copyrighted right?
Yes, it is copyrighted, and one is not allowed to distribute it. It is TTF. If it is available on the device you're probably allowed to use it in your applications. Such is the case on S60. Don't know about Maemo, don't know about using the font in artwork for on the device. I don't think they like that, nor do I think they like when you use the font in your website artwork. This is because the font differentiates Nokia from others. I advice you to either not use the font or contact Nokia Legal.