I found a page, Canada Gasoline Tax Information - Ontario Gas Prices, listing fuel taxes by province. Ontario's gas prices already includes 14.7 cents per litre of provincial fuel tax, and comments noted "Additional taxes include 5% GST." For Nova Scotia and New Brunswick, where HST is in effect, comments noted "Additional taxes include 13% HST."

That would seem to imply that if Ontario is introducing the HST, then there will be an additional 8% added (Ontario's current PST) to the price at the pump. I wanted definitive information, though, so I did a little more digging:

First, I came across Gasoline Tax in Ontario, at the Ontario Ministry of Revenue, confirming fuel tax of "14.7 cents per litre of unleaded gasoline". While it confirmed the fuel tax, it didn't mention GST or HST.

Next I found How are consumers affected by the HST?, also from the Ontario MoR. That page lists goods & services that won't be affected because they are already subject to both GST + PST, and then goes on to list those that will be affected since they aren't currently subject to PST. One item listed under "Change in Taxable Status" is gasoline:

... some goods and services that were not subject to the PST will be subject to the provincial portion (eight per cent) of the HST for the first time. ...

Change in Taxable Status

• Electricity
• Gasoline
• Heating Fuels
• Internet Access Fees
• Personal Services (e.g., Hairstyling)
• Professional Services (e.g., Legal, Accounting and Real Estate Fees and Commissions)
• Tobacco

So: I expect all-in fuel prices in Ontario to jump on July 1st, 2010, when HST takes effect.

Finally, here's one more resource I came across: Petro Canada has a blog called PumpTalk with this post: HST & Gasoline. A look at where it's already in place (March, 2009). Relevant excerpt:

... Obviously there are many details yet to be released, so this is just for fun. If nothing were to change, simply adding the 8% Ontario PST and applying it the same way as the GST is applied to today's pump price in Ontario would add 6.6 cents in tax to a litre of gasoline. That means a pump price of about 92 cents per litre today in Toronto rather than 86 cents...or about $2.64 more for that 40 litre fill-up. ...