To install Debian on a new server technology, like now the Dell R710, you always have troubles because of drivers. Always.
Solution: install your Debian with the latest drivers (and sometimes some patches) to just make it work. You find such a Debian on http://kmuto.jp/debian/d-i/
Something known, but it had to happen to me of course: on the boot of the install CD I had to add "vga=773" to the kernel options. Strange. You find that in the FAQ of http://kmuto.jp/debian/d-i/