I tend to use the .mm extension by default when creating new classes so that I can use ObjC++ later on if I require it.
Is there any disadvantage to doing this? When would you prefer .m? Does .m compile to a faster executable (since C is generally faster than C++)?
If you only use C features, .mm files should generate code that performs very similar to .m
There is also no downside to renaming a file from .m to .mm later when you desire C++ features
The major disadvantage to using .mm over .m for "normal" Objective-C is that compile times are significantly higher for Objective-C++. This is because the C++ compiler takes longer than the C compiler. With Xcode 3.2 and higher, Objective-C code can use the Clang frontend tool chain to significantly speed up Objective-C/C compiling times. Since Clang does not yet support Objective-C++/C++, this further widens the gap in compiling times between the two.
A better strategy is to use .m by default. If you need to use Objective-C++ later in development, there is no harm in renaming the file to use a .mm extension. If you so from within XCode, the project will be automatically updated to use the newly named file.
Of course all of the standard caveats apply once you try to compare Objective-C++ vs. Objective-C performance at run time. Since Objective-C++ is a C++ superset while Objective-C is a C superset, you are dealing with two different languages each with performance tradeoffs at runtime. Given that you're using Objective-X at all, you are likely writing a user-level application (not a systems level app) and the difference in performance between C and C++ wil likely be completely determined by your abilities to code efficient algorithms in each language. If you're a C++ developer, you'll likely code better than in C and visa versa. So, as always, use the appropriate tool for the job.
For reference, you may also be interested in this answer: http://stackoverflow.com/questions/1735085/c-vs-c-objective-c-vs-objective-c-for-iphone/1735666#1735666