It looks like the Append function is changing your input. Have you tried manipulating 'LIBPATH' through the __set_item__ interface? Try:
env['LIBPATH'] += ':'+libs
or
env['LIBPATH'] += ':\\\\share\\lib\\lib'
Also, if you want to get out of python \escape-hell, you can use the string prefix r which stands for "raw" and all your \'s will be passed through unmolested.
env['LIBPATH'] += r':\\share\lib\lib'
EDIT:
In response to the author's comment and in order to debug this further try:
append_lib_path = r':\\share\lib\lib'
print 'DEBUG: append_lib_path is', append_lib_path
print "DEBUG: before appending to env['LIBPATH'], env['LIBPATH'] is ",env['LIBPATH']
env['LIBPATH'] += append_lib_path
print "DEBUG: after appending to env['LIBPATH'], env['LIBPATH'] is ",env['LIBPATH']
If you see the correct value in env['LIBPATH'] on the last print, then something else in scons is mangling your input. If the string you want to append to the lib path is incorrect, try manipulating the string in the python interpreter. Do <Windows start> -> <Run ...>. Then type 'python'. This should give you an interactive python terminal and you can experiment with string manipulation. If doing that sequence of commands doesn't work, you can try to find your python install someplace and double-click the python.exe file.