never heard of mybb befopre, but out of curiosity, after reading this discussion, went and look at their site (mybboard.net).
ianal, but their license is a ridiculous bunch of nonsense, and i wouldn't even consider using any software with such a silly license.
examples:
mybb license agreement wrote:
Reproduction and Distribution
You may produce re-distributable copies of MyBB as long as the following terms are met:
* You may not remove, alter or otherwise attempt to hide the MyBB copyright notice in any of the files within the original MyBB package.
* Any additional files you add must not bare the copyright of the MyBB Group.
* You agree that no support will be given to those who use the distributed modified copies.
* The modified and re-distributed copies of MyBB must also be distributed with this exact license and licensed as FREE software. You may not charge for the software or distribution of the software.
notice the 3rd bullet: what does it mean? it can easily be read as "if you modify the software and re-distribute it, you are not allowed to provide support to the users of the modified version". it may not be the intention of the moron who wrote the license, but this is want it says. and, if they mean that no support for the modified code will be given
on their site, then it should not even be a part of the license, as it has nothing to do with the software itself. it should simply be a part of their site rules...
in other words: this requirement is unclear, but in any way you want to interpret it, it is still stupid.
even worse, lookee here:
mybb license agreement wrote: Termination
Without prejudice to any other rights, the MyBB Group may terminate this EULA if you fail to comply with the terms and conditions of this EULA. In such event, you must destroy all copies of the MyBB software and all of its component parts. The MyBB Group also reserve the right to revoke redistribution rights of MyBB from any corporation or entity for any specified reason.
wtf? practically, it means that if i use this piece of gold, someone can wake up tomorrow morning and decide i did something wrong (say, supported a user of a modified version...

) and order me to "destroy all copies" (naturally, this would include the running one) of the software, and if i want to abide by the license, i will have to obey.
as i said above, ianal, but i doubt very much that this "license" would stand the scrutiny of the law. in most probability it is completely broken, legaly speaking. however, i would not dare to take a chance on anything that came with such a ridiculous "license" attached to it.
this is not "free software" by any reasonable meaning of the word. if you'd consider using this, you might just as well go all the way and use smf...