SQL Server: are backups between same major/minor version but different build compatible among them? For example a backup executed on SQL Server 10.50.6000.34 can always be restored on SQL Server 10.50.4000.0 or 10.50.2500.0. All of them indicates an SQL Server 2008 R2 (10.50.xxxx.xx) but different build (service pack 2,3 etc).

From some tests that I did seems that there are no problems but I want to know if this is always possible.
It should always be possible to restore within the same version, irrespective of service packs and other updates. Having said that, it is also recommend that you keep up with service releases so in an ideal world this won't be an issue.

One thing to note that throws some people off is that 2008 and 2008R2 are considered different versions (not one being a service release of the other) so you can't restore a backup from the latter with the former.