Well, I guess I'll wait and see what happens with this legislation before making a CNG vehicle purchase. With the current 50% Oklahoma tax credit and the $4,000 federal tax credit (potentially reduced by AMT), it would still cost me about $3,000 to convert a vehicle to CNG and it would not be a bi-fuel vehicle. Appears that the proposed legislation would allow federal tax credits for bi-fuel, and would increase the dedicated CNG conversion tax credit to the point that the CNG conversion would be free to me!
Here's a link to an article in Platt's. The bill is New Alternative Transportation to Give Americans Solutions Act (HR 1835) - google it for more info.