I remember ages ago when I was a kid, one of the first records I bought was "Harder/Faster" by April Wine. I was shocked when I listed to the tune "I Like to Rock" towards the end, where they blatantly used the riff from "Day Tripper" by the Beatles and "Satisfaction" by the Rolling Stones. I guess it was more of a homage thing, so no one cared.

Later, when I was in a band, I wrote a song that had a bit of Edge/U2 flavor. Everyone really liked the opening riff, and I got lots of compliments on it. I later divulged that the riff had actually come from a telephone commercial soundtrack I'd seen on TV.

Its hard to tell sometimes where the line will be drawn. Look at the "Bitter Sweet Symphony" lawsuit. I agree with Jeff though. Maybe just try and ask yourself is it influencing your playing, or are you copying it? But then again, if you do something like sequencing in fours in a minor pentatonic scale... who could claim that was their "riff"?