According to YNET, Yahoo! announced that as of May 2007 they will gradually update all Yahoo! Mail accounts to unlimited mailbox sizes. I couldn't find any additional source mentioning this, but I trust YNET didn't make this up.
This isn't surprising, of course. Ever since Google introduced GMail it was obvious that at some point they would announce that the mailbox size has become unlimited. Maybe the surprising thing is that the first company to come with such an announcement is Yahoo!, who's been lagging behind Google with most of the previous advances.
This is, of course, a very welcome announcement, hopefullly to be followed by all the other main webmail suppliers.
Cool!P.S: I think that with this new development, it's time that the ISP's decide whether they want to continue supplying email services to their customers. If they do - then they should dramatically improve the level of their service (speed and mailbox size). Otherwise, they're really making fools of themselves.