According to the Sziget official Facebook profile, Smash Mouth have had to cancel their Sziget 2011 performance, along with their other August dates, citing personal problems.
This is what the message says:
Unfortunately Smash Mouth has to cancel their performance at Sziget 2011. Due to personal problems, they have to cancel their whole tour in August… Sziget is looking for a replacing act.
Let’s see who the organizers get instead.