NAS offerings are still pretty basic...
...as you can only choose which rigid path you will follow:
1. You get an EqualLogic FS7500 which is very well integrated but data tiering only works within EQL, no tiering to Compellent or any other external box whatsoever
2. You get a more expensive, faster etc Compellent setup which means more of the same: it only manages things inside, does not auto-tier with any EQL or external box although at least they offer Windows Storage Server 2008 R2-based NAS heads so at least you can utilize your existing boxes...
3. You simply get NX3000-series NAS boxes running Windows Storage Server 2008 (Vista-codebase, non-R2!) and hook up whatever you want, you can even get it in clustered HA flavor - but being a generic Windows server there is no tiering whatsoever, nothing, nada, nil.
So if you want the best of both worlds, to leverage EQL for huge capacity, simple-management iSCSI boxes (PS6510E) and Compellent for faster, auto-tiering, live volume and all the other magic they do, you're out of luck, period.
Dell really needs to get their integration efforts going: in January it's going to be a year since they've bought Compellent, they really need to show something. Even if they have nothing to show at least announce a roadmap that comes with dates ie when will all these disparate systems be able to seamlessly push data back and forth, without me logging in and doing it myself...