Mountain Lion introduces a small bug in the synchronization with Skim. I post here a temporary workaround for those who might be interested and future reference.
Inside Skim>Contents>SharedSupport there is a small bash script, displayline, which uses osascript to pass some info to Skim. Apparently, Mountain Lion changed the way variables can be passed to an AppleScript, so the following patch should be applied:
lines 42,43,44 of displayline are -e "set theLine to $line as integer" \ -e "set theFile to POSIX file "$file"" \ -e "set theSource to POSIX file "$source"" \
and they should be modified as follows: -e "set theLine to ${line} as integer" \ -e "set theFile to POSIX file "${file}"" \ -e "set theSource to POSIX file "${source}"" \
(just add curly brackets in three places). With this modification the command Typeset in LaTeX should work again. This workaround was suggested by Christiaan Hofman.
Enjoy Piero