IB is claiming Xenforo is too similar to vBulletin 4.
Considering how horrible vBulletin 4 is ... I think that is grounds to sue for defamation of character.
Silly IB, xenforo is a pleasure and your horrid software is annoying. These products are NOT similar at all.
And, they don't actually need to countersue. If they win in U.K. they get their money back, and if XF wins in Cali, the judge can order for vBSI to make payments to xenforo. Basically, Pamela has to do a GREAT job in order for their lawsuit to backfire on iB/vBSI.
The judge can be harsh if they wish. In fact, they can slap iB's hand and order them to pay MORE than what xenforo is paying for lawsuits. Don't make any expectations, but most of the lawsuits work out this way. For example, LG sued Sony over blu-ray infringement, and the judge ordered LG to pay millions of dollars in damages - guess who? Sony.
There are many better claims they could make rather than defamation. However, it is up to the fine folks at XF whether it makes sense to sue. Whatever happens, I see good things ahead for Mike, Ashley and the Barefoot One.