Yes I have. However, I don't frequent any dangerous sites much, and my computer it's terribly vulnerable. I've done full scans with MWB and AVG, and I've checked my data in/out rates.
In any case, I'd recommend against changing your MAC address because when you do get a hold of the network admins they may be rather disgruntled that you're violating network policy (almost any university that registers MACs prohibits MAC spoofing, otherwise you could just use the MAC of somebody that you saw on a different subnet).