I took the Mech PE in Oct 08 and yes, some questions are in SI and some in English Units with the exception of the HVAC depth which is all English.
But do NOT focus on this!
It doesn't matter what units the problems are in, you should know how to fundamentally solve the problem. Units are very easy to convert.
Don't get hung up whether you're looking at Joules, MW, BTUs or what not. You should know how to convert units.
Have a handy conversion chart with you just in case.
OK I'll hammer it one last time -- don't worry about it! Just focus your studies on solving problems, problems and more problems and by the time you get
to the exam, you'l have no problem.