According to FW, all Costco Amex cardmembers will begin receiving 3% rebates on gas in February:

Got this from costco magazine Feb 2008.
“Beginning this month, members with the cards begin receiving 3 percent cash back on gas purchases! These include gas purchases from Costco gas stations and stand-alone stations (such as Chevron or Shell stations that are not part of a retail or grocery chain).”

Previously, the Personal version of the card offered only the standard 1% rebate on gas, along with a 3% rebate at restaurants, 2% on travel and 1% on everything else.

Edit 1/31: I just confirmed with Amex Customer Service that the Business version will not be changing and will retain its 5% rebate on gas. Further, the Personal version of the card will begin to receive 3% on gas starting February 11. You can view the actual ad in Costco magazine here.