At least one vendor will be ready with an AM3 motherboard for Monday. Asus might have the board as well, but currently we cannot confirm it. However, Gigabyte representatives have confirmed that they will launch their first AM3 board on Monday. 

Gigabyte will have its 790GX motherboard ready for retail on Monday, and as AMD will launch the AM3 CPUs on the same day, this might be the perfect match.

At the same time, we heard that some AM3 motherboards might overclock a bit better than AM2+ versions based on the same chipset, but we are not sure if Gigabyte going to have the same experience.

Update: According to our price search engine, buy.fudzilla.com both cheaper and high end version AM3 boards from Gigabyte are available today, and you can get them here.