Peopel/Companies challenge patents usually by showing what they think is prior art. For the two big patents that center around the lawsuits, I don't believe they can find the neceesary building blocks to mount a challenge or they would have done so. What they did not infringe on the patents..the same direction Samsung took but a slightly different route. This was all pointed out by Doni in great detail as he attempted to explain the filings(Pacers). It really comes down to either a settlement before the Markman or the judge's ruling. The other side does have some interesting points.