Getting rid of the m67665657 type mailbox names is our most highly voted suggestion and has been for quite awhile. We do fully intend to do it when we can do so safely. We have done a lot of research on the changes we need to make and even attempted a test switchover at one point that went disastrously. If we were to start up from scratch right now it would be trivial to do but switching over our many many thousands of mailboxes transparently will be a bit more difficult.
Also, as I have seen many complaints about our email service on these boards I'm sure you would rather we be putting our available email resources into stabilizing things before making a major change that could disrupt things.
Anyway, just now that it IS on our todo list and it's just proven to be a more elusive target than anticipated. We are in the process of doing some mail server upgrades that we believe will bring us one step closer, though!
- DreamHost Head Honcho/Founder