My contact said that the DIR-855 will support dual-band "concurrent" streams and divulged one other tidbit about the product. The DIR-855 will have an OLED display, similar to that on the DGL-4500.
He wouldn't confirm whether the DIR-855 and DGL-4500 share the same main board, which can support two radios via its two mini-PCI slots (see for yourself in the slideshow). We'll just have to wait for the FCC registration, which hasn't happened yet.
6/12/2008 Update! Read the full DIR-855 review.