Transparent icons are working just fine for me, Andy. I'm using them on both my GO 1535 Live (5" screen) and my Via 1605 (6" screen) at 25 (easy to change a 24 to a 25! I'd recommend that instead of 24 to the OP) and 44 pixels respectively.
At this moment, I'm unwilling to pull either unit out of the car for a demo shot (it's so cold I'd never get them to stick to my dash again!), but I should be able to scare up one or two here somewhere from past bug reporting...
The first is off my GO 1535, taken in October. The screen dimmed when taking the shot, but you can clearly see that the two smiley icons with the red lines across them have no frames. However, the red lines have a tiny bit of blue to make sure they stay red and don't 'disappear'.
The second is off my Via 1605 which happened to be in 2D mode at the time. Numerous custom POI icons on this one, all 'transparent' as expected.
I'm attaching one sample of each of the 25x25 and 44x44 here for you to play with just in case.
I agree, though, that they don't show up as transparent in the POI category list. Doubt they ever will!