structs (such as LLUUID) are considered values by mono. comparing them against null makes no sense and the mono compiler will flag that as evaluating to always false --- except if "cleverly" disguised. the attached patch fixes such an occurrence in OpenSim/Region/ScriptEngine/Common/ScriptEngineBase/AsyncCommandPlugins/SensorRepeat.cs.
[yes, i'm on a crusade against comparing structs against null, go ask jradford from libsl ;-)]