The C standard library seems to alternate between uppercase and lowercase identifiers for typedefs. On the one hand you have FILE which is an opaque type for storing the state of a stream, and on the other you have various numeric types like size_t, time_t, clock_t, etc. The choice of style seems to vary based on what the typedef is for, rather than it simply being a typedef.
Personally, if an identifier's a typedef I capitalise it (e.g. Foobar), if it's an enumeration member or a macro I make it uppercase, and I just use lowercase for everything else.