There ain't much details available but it looks like it will support mixed cards.
We are unclear if it is across different series or different models of the same series.. Also there are some rumors out now that GeForce 8x00 series will be able to support multi-GPUs SLI by linking up 4, 6 to 8 cards or more. This multi-GPUs SLI capability could form part of the SLI 2.0 technology we are hearing.