When the first pictures leaked of Dell Thunder, Dell got quite some attention from the tech world. It seemed like people were ready to accept Dell as a smartphone manufacturer because those phones renderings that got leaked looked darn good.
With the launch of their mini-tablet, the 5" Dell Streak, Dell showed some promise, but one thing disappointed about their tablet - that it came with Android 1.6. If the Streak would've come with Froyo, it would've gotten a lot more buzz and sales. I think they've realized this, so even if the Thunder smartphone is ready to ship right now, they will probably choose to wait a little longer to get Froyo ready for it.
The big boys, HTC and Motorola, are already deploying Froyo on their smartphones, so it wouldn't be a good move to launch Thunder with an oudated OS, and then wait a couple more months to update it to Froyo.