I'm not sure which portable you're talking about, but the Karma's manual mentions crossfading as a feature designed to seemlessly transfer from one track to the next. I just called my friend who owns a Karma, and he confirmed that Ogg Vorbis files (which are natively gapless), do in fact play back gaplessly on the Karma.

The specific transition discussed was Tool's Parabol to Parabola, from the Lateralus album. It's one of the more difficult gapless transitions I've had to deal with using gaplessly encoded MP3s on my empeg.