The Third Season of BSG will not be released on DVD until April 2008. The same month the Fourth Season is schedule to premiere. So far no one associated with the show will provide any details for the decision for the delay.
However, speculation is that NBC/Universal wants to give the TV movie Razor time to be released on DVD so they can maximize their profits. Another reason for the delay could be to add the current web episodes and other extras on the Third Season DVD while creating additional buzz on the Fourth Season and final season. Capitalism at its best!
My hypothesis is that NBC/Universal is delaying the release of the DVD due to the pending Writers Guild strike. If the strike does occur like everyone in Tinsel Town is predicting, it could be one long strike. So NBC is looking at using BSG for their network programming while the strike is going on. If so, then the Third Season would be the logical choice to show. Then again who knows!