FWIW, the POSIX standard gives some guidance on how PATH is to be interpreted for conforming systems, including:

"A zero-length prefix is a legacy feature that indicates the current working directory. It appears as two adjacent <colon> characters ( "::" ), as an initial <colon> preceding the rest of the list, or as a trailing <colon> following the rest of the list. A strictly conforming application shall use an actual pathname (such as .) to represent the current working directory in PATH."

http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html