That is why I would like to ask whether it will be available for purchase in the Ubuntu Software Centre.
If it will be offered, I would wait until it is available and buy it to support Ubuntu. If not, I'd rather buy it now and save some money.
I think it will, eventually, there is a post in Ubuntu Brainstorm to add a few more paid apps to the Software Center. Besides, Oilrush is a huge release as a Linux game, there are few games that can be compared to it, and a lot of people are looking forward for a stable Ubuntu release.
I think i got your definitive no, i'm afraid. The Feature Freeze of the Natty development cycle was on February 24th. This means that no new packages will be added to the official repositories. AFAIK, unless they make an exception, we will have to wait until Ubuntu 11.10 to be able to purchase Oilrush from the software center.
More info here