IMAP / iPhone & "On My Mac" folders?

[Casper TFG]

Hi folks

I am on IMAP (for my 2 main accounts)
I also run a Google mail account and a .mac account too

I like to keep my inbox clear by dragging completed items to a separate folder.

I just got my iPhone so am struggling on my first day to understand

Normally on my Laptop - I would drag completed items to a folder I created in the "ON MY MAC" area in Mail...

But that folder (its a big one!) hasn't been duplicated (that I can tell) on my iPhone when I synced all my mailboxes and rules via Mobile Me..

(I just got a 3GS today)

Any tips for how to manage this

I just got a bunch of old February emails in on my Laptop which was bizarre..