No Patents
PBooks has no patents, and there are no plans to apply for any. My personal goal as the current steward of PBooks is to create a piece of open software which is able to flexibly accomplish the concepts and functions present in the practice of double-entry accounting.
Prior Art
Since the practice of double-entry accounting has been around for several hundred years, I don't think there would be a chance of someone trying to patent this idea. However, like the law in general, patents are a practice, and as such they can be practiced well or practiced poorly, and depending on the performance of the practitioner, a patents enforceability can vary. Furthermore, the concepts of online bookkeeping haven't been around for too long, and its very possible someone has a patent for that idea.
Big Money
All that being said, from what I've read in the news, it appears that the mere practice of patent drafting is a very expensive act. On top of that, patent litigation can be extraordinarily expensive. Logic might suggest that patent trolls and lawsuits would target the "big fish", as we've seen in the cases of SCO vs. IBM and now IP vs RedHat and Novell. As it is hopefully abundantly clear, PBooks is a very small fish compared to those 800 pound gorillas! Nevertheless, it is difficult for me to say whether or not the use of PBooks would be subject to any patent infringement arguments.
Ask A Lawyer
The only logical thing to do if you are unsure about the legal implications about the use of open source software is to consult a legal professional, which I am obviously not.
Side Note
In the open source community, I feel that there are a lot of sour attitudes toward software patents. In many ways, the idea of "thought police" is unsettling. Since many open source projects, like this one, are run by individuals, its hard to conceive why a patent would be worth the effort.
In my opinion, patents make more sense for large corporations with very good reason. I can imagine them as a risk mitigator for the investment of time and effort of several orders of magnitude greater than an individual is physically capable of producing.
On the flip side, patents can and are being used as a method of intimidation towards and against individual innovators, who ironically have the most creative freedom. The idea that a corporation can draft a blanket patent (valid or not), use that to put a fledgling startup out of business with unbearable legal fees, and effectively absorb their innovations is definitely depressing! That's why I believe efforts like the new online peer review system are very important steps in the right direction.
http://www.peertopatent.org/
The use of documentation in legal disputes also compels me to try and document as much about PBooks as possible, so there are no misinterpretations about the scenario.