There are mobo with both ddr and ddr2 slot but I am not sure if you can mix them up. Search through the forum, I am sure someone already asked that question before. Also those board are quite hard to find.
Some motherboards support different RAM types, either through a Riser Card (Asus P3C-E has this, Supported RDRAM and PC-133, RD-800 RAM with a P-III tualatin proc was fun times, lol) or separate DIMM slots (I've seen quite a few interesting boards with this, especially when a new RAM type is being introduced to the market). I have not seen an actual motherboard/chipset which supports simultaneous DDR1/DDR2/DDR3/etc. memory operation, I've personally only seen either/or operation, however, this doesn't mean it does not exist though.