Punk shouldn't turn face yet because his upcoming role with Undertaker works best with him as a heel. Same with The Rock previously. He did a very heelish thing (ok, not really, but WWE played it that way) by attacking The Rock and that feud worked a lot better with Punk portraying the heel, especially with Heyman in his corner these past several months. But I get the feeling the Punk/Heyman relationship is starting to run it's course.
One very likely idea (that I've mentioned before) about how dear Punk could turn face is he makes it clear to Heyman that he wants to take on Undertaker and defeat the streak all by himself without Heyman's help. He fails, but he puts on such a performance and comes so close to ending it (close enough that people who easily dismiss Punk ending the streak even start to slightly doubt themselves at a couple of points) that people show a huge showcase of respect for Punk, and so does Undertaker himself. Meanwhile, Heyman (if Punk where to reject his help during the match) bereates him the next night for refusing his help, and he has Lesnar destroy his ass to send him off on a (kayfabe) injury for a few months.
Either way, Heyman dumping Punk or vice versa, especially with Brock Lesnar (Heyman's 'first love' between the two, so to speak) wandering about, is the obvious set up for a Punk face turn. Don't forget the rumors of a Brock/Punk feud later in the year, which should be awesome. (Remember, it was only Meltzer who assumed that the baby face in that feud would be Lesnar.)
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