Treasury stock or new shares does not matter, well not for US companies. GAAP accounting rules dictate that treasury stock is taken out of the float. It is shown on the financial reports, but it is not counted in the market float.
Issuing shares vs Treasury stock is really whether they need board approval and stuff like that.
So the treasury stock thing is a non-issue.
From the press release it sounds like they are not going to fully value the deal on the books. Which is smart, because there is political risk.
So technically there will be dilution, but only because the way they do the accounting on the deal. Will it impact the market value of the stock. Nope.
Honestly I would worry more about the spot price of crude impacting earnings than anything else.
I'm not good at "trading" stocks, I tend to take a 3-6 month view. I'm not currently a holder. Having said that, everything being equal (eg chiefly political rethorics above and beyond the saber-rattling "we won't issue any more licenses to BP" or "fines won't be tax-allowable"), I don't see how you would get a pull-back on the ADR closing price. It's a win-win situation, which means that you won't get a share arbitrage one up one down. Strategically, that gives BP a new playground away from the inclement weather that had dominated in the GoM. That's a big plus IMO.