I've read some discussions about this, and they all tended to boil down to "nobdy knows for sure, more testing needed".
However, it's been generally agreed that a preponderance of links to domain.com will give domain.com a higher PR than
www.domain.com.
The general rule, as I learned it, is to pick a version of the name you want to rank, and try to use a consistent version of the link pointing to that name version.
So it would be typical for domain.com to rank higher than
www.domain.com if most of the links pointing to the domain were in the form domain.com.