Yep, you can breed the starters. As Br26 has stated, there are a lot of different combinations for breeding, but unless there's a specific move that I'm trying to breed onto the new Pokemon, I usually stick with Ditto, as it can breed with any type of Pokemon that is able to produce an offspring.
If Ditto's unavailable in Diamond and Pearl, then it's just a matter of trading them from Leaf Green / Fire Red. I'd be more than happy to send out a few if some of you do not have a copy of the GBA games, when I progress enough to enable Pal Park.