Hi Steve!
That's odd. It should work perfectly well then.
One thing that comes to my mind: When the sound ID of the instrument you choose in Sibelius and the Sound ID of the program you choose in the Manual Sound Sets tab are not the same, keyswitches and control changes often do not work correctly. For example the Sibelius instrument "Solo Violin" has the Sound ID "strings.violin" and the Program "Appassionata Violins" has the Sound ID "strings.violin.ensemble". So these two combined will not work properly.
You can find (and change) the Sound IDs of the Sibelius instruments in House Style/Edit instruments/Edit instrument/Best Sound.
If all articulation changes work well and portamento is the only one which doesn't, the instrument Sound IDs are probably not the problem.
If you want to, you can send me your Sibelius file and your playback configuration and I take a look at it.
You find the playback configuration at the following location:
Documents and Settings\user\Application Data\Sibelius Software\Sibelius 5\Playback Configurations
It consists of an xml-file and a folder with the same name.
Best,
Andi