Even if $600 system with Blue Ray is cheapest device right now but it's kind of taking a chance. We'll never know what will happen in the future that what format will dominate the market. Blue Ray could be next DVD revolution or HD-DVD maybe take over or something else. Even if Blue Ray is become a stardard and by the time all the rental company carries BR disks for all the movies, Blue Ray player will be dirt cheap.
It's like any new technology, if you wait, price will go down really fast. CD recorder first came out and it was priced at over $1000. Within 1 year later, by the time CD-R blank disk was affordable to everyone, CD recorder was like less than $100.
What's a point of paying highest money to get the device which I will hardly ever going to use it. And by the time I'll get to use it more often, Blue Ray player probably get dirt cheap.
I think XBOX is taking the right course. Use whatever it's available today and you can upgrade it later. And it's probably going to cost less even if you have to add new Blue Ray or HD-DVD device to your current XBOX360 since those device will be very cheap and easy to come around.