If the developer has commented out some code because it is not yet complete, then the correct "source control" way to deal with this would be for that developer to keep it in a separate branch of his own, until that code is ready to check in.
With a DVCS (like git, bazaar, or mercurial) this is dead easy as it requires no changes in the central repository. Otherwise, perhaps you could talk about giving developers their own branches on the server if they are working on specific features that will take them a long enough time (ie, days).
There is nothing wrong with checking in commented out code in some situations, it's just that this is one situation where there may be a better way to do it, so the developer can track changes to his source even though it isn't ready to be checked in to the main repository.
