Updates are free, but further updates are highly unlikely. I bought it early on and there have been 2 updates I think, almost to coincide with Pr1.2/Pr1.3 but I could be wrong.

You will get an extra repository on your device for updating, so the updates are delivered OTA like any other.

[edit] The current version for N900 is v1.2.2, and it doesn't work with the overclocked kernels - although there is a manual workaround you can use.