Hi Dan,
Matt emailed me a while back asking for help with this and i have to admit the email got lost in my inbox before i could add it to my task list. i am really sorry for keeping you waiting, it doesn't happen often and when it does, its a genuine mistake.
it is possible to center menu items, the only catch however is the menus list items and links cant be set to display: block; - which is unfortunately required in the suckerfish menu.
The other option is if your menu items are set (ie, not going to change) then you could add a fixed with on the menu UL and then center that with margin: 0 auto;
Both methods are a bit limiting, which is why we try and avoid them where ever possible.
I hope this helps and my apologies again for the long delay
Cheers,
Chris