If I get Chain grease or pick up any oil spray from the road I use Orange power from Bunnings, It is designed to clean marks and stains off walls at home, but without damaging the paint.
I have used it on every surface on the bike with out any effects or marks, I don't normally spray it directly on the surface unless it is a heavy buildup of Shite, I normally apply a little to a cloth, and then a quick wipe and the grease or oil just comes straight off.
If I do spray it on directly, I agitate using a soft brush, then hose off and then wash the bike as normal, I always wash the bike afterwards as the Orange power does leave behind a slight film.
It's also gives your bike a spiffy Orange aroma!
http://www.orangepower.com.au/prod_MultiPurposeCleaner.shtml