I'm using one (three actually, one per plane) of the NovaTech alarms... cheapy things (less than $20) that plug into the aileron channel and have the aileron plug into the alarm's output, with a Spektrum DX7 tx and Spektrum AR7000 or AR9000 rx's. The alarms aren't specifically for 2.4ghz, but I've had no problem with it, except for when I tried plugging two aileron servos (with a Y adapter) into the alarm's output and the ailerons went crazy when the engine was running. Problem disappeared when I put the Y adapter before the alarm and only used one servo on the output.

They also double as a low voltage alarm, but you've got to hold right aileron while turning the plane on to activate it, and 2.4 usually takes too bind for the low voltage trick to work. Oh well, at less than $20 it's cheap insurance.