Okay, I know this sounds stupid, but I absolutely can't figure out how to set my ring tone on the N900.

I see the ring tones folder in my home directory with tone mp3 files in it but I can't find any dialog to change from the default Nokia tone theme.

you go to settings->profiles there are 2 profiles. silent and general. if you click one and scroll down you can enter tones for ring tone, IM tone, sms tone and other things like touch screen sounds.

Beat me to it Cherry.

Settings > Profiles > Select "Ringing tone", "SMS Alert", "IM Tone", or "E-mail alert". You can also "add" a new ringing tone or message alert tone.

Here's the User-Manual, if you don't have it! It explains it on there as well as everything else

I have downloaded several ringtones from some apps that search for them. I can change my ringtone by person calling or just the general one just fine. What I am trying to do is change the tone for texts or IM and the other ones you can change but my only choices are the default tones and not any of the ones I downloaded. Any idea how to change those to the downloaded ones as well?

just go to your media player... open songs....long click on the song you want as ringtone....you will get an option to set it as ringtone.....select it without taking your finger off the screen....and everyting is done

I am hoping it would have been sorted by now.
2 and a half years later....

How can we assign different ringtones to different incoming numbers?

There is 'Custom ringtones for your contacts' (per-contact-ringtones) in the repos. I never tried it though.

afaik it's quite buggy and it takes ages to actually hear someone is calling you (when someone's calling, the old ringtone is disposed, new one is loaded and played - it takes time), so it's not really a solution.