Hi,
There sure is. Multivalue parameters are arrays so the best way to do this is with IndexOf. e.g.
=Array.IndexOf(Parameters!Param1.Value, "myvalue") > -1
Array.IndexOf takes an array (your parameter) as the first argument and the value you want to look up as the second argument.
It will return a 0-based index of the value if it exists in the array and -1 if it doesn't, which is why you use > -1 as the condition.
HTH
(also to save you from RSI, Parameters!xxxxx.value = "xxxx" would return true or false anyway, so you don't need the IIF)