There's a good multilingual website with suggested cycling routes in the Luberon:

http://eng.veloloisirluberon.com/

I followed most of the route "Autour du Luberon" in 2010 and enjoyed it a lot. I occasionally varied from the suggested route to visit specific towns. It is well signposted and mostly is on small roads with little traffic, and occasionally farm roads or bike paths. It is NOT a flat route. There are a few significant climbs in the eastern part, though as is usually the case in France, the grading is good. Tourist offices in the region have free brochures/maps for this and the other suggested bike routes in the area.

The northern part of the route goes through a more populated area. The southern part is more rural. My favorite parts were the middle section of the southern part, and the area east of Apt in the northern part, and also southwest of Apt. It's not particularly great around Cavaillon at the western end, but you can move away from there fairly quickly. That's the flattest portion.

I've biked around the Gorges du Verdon, too, but I liked the Luberon more.

The area can get quite hot in the summer. I was there in late May and it was quite hot for that time.