There is actually a trick whereby you can do this. The thing is to change the "colour" of your desktop background (the colour it would show if you're not showing a picture on it)
The easiest way I know to do this is by using firefox (not tried it with other browsers). Basically, open up the image you want to use in firefox (I find all of my desktop backgrounds on the internet anyway), then right click on it and select "Set as desktop background". On the dialog that then shows, choose between Stretch/Tile/Centre and then on the top right there's a selector marked "Colour"
Set this to the colour that MOST RESEMBLES your background, not that which you'd like your text to come up with. So, if you have a dark background, set it to black, if you have a light background then set it to white.
Vista will then change the colour of your icon text accordingly. If you set "Colour" to white, then the text will appear black, and vice versa.
I just tried setting "Colour" to red and the text has stayed white, so maybe this trick will only work for changing the colour from black to white and vice versa. But at least now you can choose a little