Can I hook up a MyBook Thunderbolt Duo external drive (only has Thunderbolt ports) with a Thunderbolt cable to a late 2009 iMac (with mini DisplayPort) and use it for storage (at a slower speed) until I can buy the new iMac comes out and I use it with that? Will the data still be accessible then?
Nope. Mini DisplayPort has a physically compatible connector to Thunderbolt, but it's a totally different technology. You can only use Thunderbolt devices with Thunderbolt equipped Macs (or PCs).