the 780G is ideal for HTPC use, the 790GX adds better overclocking and crossfire for mATX gamers. The proposition is lose / lose in a market that caters to HTPC users. You are asking for them to spend money to release a board that will take away sales from an almost identical model already available to cater to a small enthusiast market...
That being said, I applaud Biostar and DFI for releasing mATX 790GX boards and according to an email from motherboardpro.com, they should have a US release date for the Biostar TA790GX XE tomorrow (1/5/09). I plan on buying one and adding a 4870X2 once prices drop so I am not really interested in DFI's LanParty JR board.