2011 iMac - external display won't go into powersavings?

I just hooked up a Dell 2405FPW to my 27" iMac, and everything seems to work okay -- except when the iMac tries to put the display's to sleep, the Dell goes into "Can't display this mode"

Haven't tried another display yet, but was wondering if there was a workaround or if this is a unique problem.

Using a monoprice MDP -> DVI adapter, which seems to work fine for actual video signals.

 

Apparently this was an issue with the Dell - I swapped with my wife's Acer 24", and it goes to sleep just fine.