Meguiars Hot Rims All Wheel Cleaner.....
I used this product today....and can wholeheartedly recommend it.
I've been neglecting the front rims of teh VXR recently and they had gathered a fair anount of brake dust and grime.
Shake this stuff up, spray it on, leave for 15 secs (yes only 15 seconds ! )
Then hose it off using a powerful jet of water (the hose was sufficient).
The jet just washed most of the grim off - amazing. I repeated it to get the long term stuff of, but could have wiped and rinsed because it loosened the grime suitably.
Buy some - it Rocks !