That's been one of the reasons I've not donated either (apart from one £10 donation made for odd reasons). Unfortunately, the SIAI seem to have made a decision that they need to keep what work they're doing secret. That's understandable, but it means we have absolutely no idea what the actual effect of donation is.
Holden Karnofsky of GiveWell.org interviewed Jasen Murray of SIAI and published his notes (Edit: PDF, thanks lukeprog!), with updates from later conversations. Lots of stuff to take an interest in there - thanks to jsalvatier for drawing our attention to it. One new bit of information stands out in particular: