The effect on the beer would depend on the length of the exposure, but it is not definitive as there has been no proven research on the topic. The definite factor that will have detrimental effects on the beer is exposure to light. This will result in 'skunking' due to exposure of isomerized hops to light.
I think its true for American beers because most if them have CO2 added to give them 'body' and a good 'head'. Heat kills the fizz and it cant be regained. In europe i believe beers and ales are often uncarbonated and fequently served warm or at least room temp.