I have recently been offered place for PhD. But it will not be possible
to meet September 2006 deadline for registration. Therefore I
defered my registration to January 2007. Now, Do I have to wait until January 2007 for registration? Or If I want to register, Can I register in October 2006 and begin study in January 2007?

err, why would you want to do that? As soon as you register the clock starts ticking to when you have to submit your thesis, there is no point in registering and then NOT studying!

You could do it the other way round, start studying (unofficially, eg doing background reading etc) in October and register in January.

You'd be better off asking someone at the postgraduate office at your University.