Mavericks not able to use external display at full resolution
Finally I found this. Looks like the default EDID configuration for Dell displays is not quite right.

On that post there's also a ruby script that suposedly generates the right file for your display, but it ended up screwing even more my resolution.

I downloaded the file from the first post, and after copying it to /System/Library/Displays/Overrides and rebooting, my display was showing again its awesome full resolution.
The files are suposedly to correct issues with U2713H and U2713HM, but it worked with my U2711 also.

Hope this helps someone as it helped me.