If you want kernels I would not hold out on the Atrix, it has an encrypted bootloader. This will be very hard to hack, but if anyone could do it it's the devs on XDA. If you want something on ATT that will give you lots of development options go for the inspire or buy another captivate. I bought the inspire and i like it alot. But you don't get the dual core and some other features that the atrix has. I guess you gotta just know what you want, either development or better specs. For me I will always go for the development.
Inspire 4G-Some AOSP Rom
Xoom-Latest Titamat kernel