Joa Ebert is questioning on his blog whereas having an open source decompiler/obfuscator software would make sense.
I can maybe help a bit with this choice, since I was confronted to the same one a few years ago. At this time I wrote an AS2 decompiler as well as a SWF obfuscator that we are still using at Motion-Twin.
People that have met me know that I'm a big proponent of open source and open protocols, so why didn't I open sourced these software after writing them ? There are good reasons behind these choices.
While decompilers are a very good tool for the expert to learn about the way a given compiler can optimize code, in the hand for the average user they are mainly used to look through other people code.
I'm all for open source and code sharing, but I think that someone should get the original developer agreement before looking at his code. I don't consider that decompiling is the same as stealing, since the original code is not lost in any way (this is somehow similar to downloading an MP3) but in practice, you will often find people decompiling for the sake of just copying, only because they are too lazy to solve a given problem or because they have a deadline and just don't care reusing someone else work without asking.
While an open source decompiler is a great tool for hackers (but in that case a disassembler is already quite enough), it becomes an immoral tool in the hands of mister everybody. So I guess that from a moral point of view, an open source decompiler would not fit any particular "good usage".
Somehow the entire opposite from the previous one, since the only usage one might have from an obfuscator is to protect his code from being decompiled.
Some side story : in early 2009 I have been working on an Flash9 obfuscator prototype that is working nicely. We thought to sell it by making a partnership with PowerFlasher (the company behind FDT IDE) but we found that using the software was hard and required a good knowledge of the theory being a virtual machine, as well as many code changes in some cases. We then recently decided to not push further the project, although we will be using the obfuscator in-house at Motion-Twin for our Flash9/10 projects.
So yes, I have no problem with selling an obfuscator software. Since I think that it should be the developer choice to either share its software or not, one should also have the choice to protect himself from people using decompilers.
However, I'm having a problem with the idea of an open source obfuscator, since the two ideas of "open source" and "protecting your code" are clearly opposite.
Also, I don't think that people should be encouraged to protect their code, and I don't think that someone should provide them an easy+free way of doing so. Because the best people feel their code is "secure" the more they will be sensible to their code being "reused", even if it's not at all the same code but the same behavior or same principles.
I think that developers in general should first focus on improving their skills instead of thinking about protecting the things they have been written and that almost nobody would actually like to copy.
Obfuscators and similar technologies are putting too much emphasis on the notion of property of code, and since I'm also a big opponent to software patents, I don't want such ideas to become mainstream : we don't have software patents in Europe and this is something that we should really defend.
Of course obfuscators don't have anything to do directly with software patents, but at the same time first someone might want his code to be protected, then later he want not only that, but also the ideas behind his code to be protected. And that's where software patents start being a problem.
There are still a few valid reasons to use obfuscators : for instance we are mainly using it not to protect our games implementation (because we don't care) but to protect the protocols used to save the score or exchange data between client and server, since we don't want these to be exploited by malicious players that would ruin the fun of other players (such exploits are not possible in secured games when the server handle the gamelogic, but for arcade games you don't really have the choice).
In conclusion, this is not because your are making an open source software that you are doing a "good thing". There are some cases where helping a software go mainstream can actually go against what you were thinking at first. Open Source and (sense of) Morality are two different things, but when combined they become extremely powerful.
And I guess that's all I had to say on the subject :)