Here's what Snopes has to say about it:

http://www.snopes.com/politics/gasoline/nogas.asp

This year's e-mails (proposing a one-day "gas out" in May 2007) is yet another recasting of similar messages that have been circulating since 1999. All of them are reminders that "protest" schemes that don't cost the participants any inconvenience, hardship, or money remain the most popular, despite their ineffectiveness. A one-day "gas out" was proposed in 1999, and a three-day-long event was called for in 2000, but both drew little active participation and had no real effect on retail gasoline prices.

The recycling of the same campaign in subsequent years hasn't drawn enough interest to even be considered newsworthy.

The premise behind all these messages is inherently flawed, because consumers' not buying gasoline on one particular day doesn't affect oil companies at all. The "gas out" scheme doesn't call upon people to use less gasoline, but simply to shift their date of purchase and buy gas a day earlier or later than they usually would The very same amount of gasoline is sold either way, so oil companies don't lose any money.

There's more at the link. Snopes is the best debunker of chain emails and urban legends on the 'net.