This is one feature I'm glad they're stealing from Rock Band. Although the "some" part makes me wonder just how many of the songs from World Tour and Smash Hits will be importable. Another cool addition is that Johnny Cash will be a playable character.
From IGN:
Guitar Hero 5 will be the first in the franchise to feature forward compatibility of songs from past games. On PS3, Xbox 360 and Wii, some of the disc-based songs from Guitar Hero World Tour and Smash Hits will be importable, for a fee, to work with Guitar Hero 5 and the upcoming Band Hero. What's more, currently 152 out of 158 Guitar Hero World Tour downloadable tracks will automatically work with Guitar Hero 5. All of the songs you import will be upgraded to work with the new Guitar Hero 5 features, such as band moments and vocal star power.
http://ps3.ign.com/articles/101/1012173p1.html