I'd just started using the Subversion bundle, and everything was working great once I defined $TM_SVN except for the Log command, which would complain that it couldn't find the svn binary.
Turns out that the problem is within the format_log.rb program; the line defining $svn_url read as follows:
$svn_url = `svn info #{ENV['PWD'].quote_filename_for_shell}|grep URL| cut -b6-`.chop
changing it to:
$svn_url = `#{ENV['TM_SVN']} info #{ENV ['PWD'].quote_filename_for_shell}|grep URL|cut -b6-`.chop
corrects the problem.