Harmonix appears to be taking a break from Rock Band Unplugged releases, moving away from a weekly batch of downloadable tunes for the Frequency-esque PSP spin-off. But that doesn't mean it's ditching the portable rhythm game.
Joystiq reports that Harmonix is diverting its resources toward Rock Band Network deployment, downloadable content for the console versions of Rock Band and "unannounced future projects." That "future" may include more Rock Band Unplugged-tailored tracks, just don't count on a weekly refresh.
In the meantime, have you missed any of the already available tracks? Sounds like you'll have plenty of time to catch up if you did.