Why would you want to store PMs?
User limit is 100?
Surley, you cant have 100 PMs that you want to keep???
I am actually thinking about decreasing the amout of PMs allowed to be stored in the near future to 50 just so that you are all away.
IMHO there is no need to have more than 50 PMs in your inbox - Its just a fact people are too lazy to delete them
I bet half of everyones PMs contain just "Thanks mate" or "Yeah thats cool" or a link to a website, etc, etc.