ummm... einstein by excalibur electronics ;)
any good portable chess computers out there?
The most recent sony vaio with rybka might challenge you for a while.
Seriously though, I don't know. I've got some kasparov model by saitek from the nineties that's annoying as hell, takes years for it's moves if you want any challenge and all in all, I'm not sure if it constitutes as 'portable'. But it only cost 10 euros in the local bookstore, since they were trying to get some space to their shelves, so can't complain I guess.
...with club strength and above?