Matt Cutts from Google explains how they treat pipes (|) and dashes(-) and there use in meta titles.
Essentially, he says it makes no difference to their engine, which you use in your meta title. For example if your title looks like this (on SEOmoz):
SEO Blog | SEOmoz Blog Featuring Search Engine Marketing & Tips
SEO Blog- SEOmoz Blog Featuring Search Engine Marketing & Tips
Makes no difference to Google. However, one may want to consider the aesthetics and watch the click-through-rate. We like using the pipe |, just because we think it looks really professional. It’s nice to hear directly from Google that they handle each really well.