One problem related to engine overheat has been reported for the 2008 Mini Cooper S. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2008 Mini Cooper S based on all problems reported for the 2008 Cooper S.
Found large oily mark under parked car. Turns out to be the thermostat hose leaking due to cracks because of being made of plastic. This is a known design issue by Mini/BMW that could cause an engine to overheat producing a file or blowing the engine. This should come under a recall. Checking online, you can see that it was such an issue last year, that dealers were not even able to get parts for weeks due to the number of failures. Calling the dealer, they readily admit it is a huge problem and they have to replace lots of them. Of course, it is $600 a pop for the fix. Due to the large cost and admitted design flaw, these should all be placed under a recall before someone is badly hurt. Also anyone that has had to have this fixed should be reimbursed for that expense.