When going above this temperature numerous reliability problems pop up, some of which are:
-purple plague at aluminium bondpads with gold wires (see
http://en.wikipedia.org/wiki/Purple_plague)
-electro-migration in aluminium gets very problematic (
http://en.wikipedia.org/wiki/Electromigration)
-NBTI and PBTI can get very high (
http://en.wikipedia.org/wiki/Negative_bias_temperature_instability)
Some none-reliability problems:
-leakage can get very high
-transistor models might be less reliable (they are not tested)
Usually the design manual specifies reliability at different temperatures, and usually 150 C is a lot worse than 125 (if it is specified at all, a lot of processes are supposed to stay at 125 C maximum).
The 150 degrees limit will be based on history, it is not a hard limit but common process qualification stops at 150, so above that you'll have to re-qualify the whole thing. It might also be very hard to get it pass unless it is only at 150 C for a short time, and you might get unknown reliability problems.