The Milestone's processor is slightly underclocked, it can go up to 600Mhz. I also went through the process of investigating the processor specs between the Milestone and HTC / Nexus a while back and the Milestone's Cortex platform appears to have a better GPU which helps speed up the video bits and lightening the load on the CPU. So while it sounds like it's 400Mhz less, the user experience is fairly similar between the two.
I bought one in Jan and I love it. Great phone, feels really snappy and the battery lasts about 2 days for me which seems to be a day more than the HTC / Nexus. These phones have been available from Expansys SA since Jan, imported via UK.