Does anyone know where I could find really light 24 hole tubular rims like the Mavic OR7 ( about 270 grams ) or even lighter?

Tune do a pair of Zipp rimed wheels that are based on the Zipp 303 rims, apparantly they are special versions made for Tune and weight in at about 245 grams

lightbike used Zipp Z 3 rims, one of the lightbike guys mailed me a few days ago. not sure you can get those as rims only when you are a mortal, though, because zipp normaly only sells them in the complete Z 3 wheelset.
usually tune uses the standard zipp 280 rim for the tune olympic gold, they weigh in at approx. 285 grams.

Yes they use the normal 285 gram rim for the tune olympic gold which wieghs in at about 1080, but if you look at their site they do the skyline edition which uses a much lighter rim, it states that the set is garaunteed to come in at under 1000 grams. I have it on good authority that the sets average about 910 - 920 grams and that the rim weight is about 245 for 700c