I also have all of my Xmas music as "holiday" genre. I too have a smart play list that finds all of these songs. What I do different though, is that I uncheck the Christmas songs when it is unseasonable.
I suppose you could do the same for songs that you consider to be more summery, or related to any other time of the year.
Un-checking them keeps them off of my iPod, saving the precious space for the other songs I want to hear!
One problem with building it into iTunes 6.0 --> how is a computer song supposed to know that a certain song should only be played at Christmas? Sure, if it includes Santa in the title, that would be a dead giveaway, unless there are songs out there that talk about Santa but arent really holiday songs:
Example: The Christmas Song by Dave Matthews - sure, it talks about Christmas, but isn't really a Christmas song that can't be played the rest of the year... smart play lists can only be so smart before they are downright scary!!
(still a cool idea though!) :-P