I've just installed an Intel 82599 based NIC with 2 SFP+ ports in my server. Right now I have a PowerConnect 2824 switch, which has 2 1Gb SFP ports.
I tried linking them with a 10Gb SFP+ DAC (a 10gtek, bought as compatible with Dell), but there was no way I could make it work. The Switch always showed the port as down, and so did the card.
I was not able to change the speed in the Card to make it 1Gb, so I guess the cable is only good for 10Gb, although in the specs it says it handles 1-10Gb.
This is only temporary: we are waiting for a new switch (Dell X1052) that has 4 10Gb SFP+ ports, where I hope everything will work fine, but I was trying to setup the link already with the existing equipment if possible.