OK it's a long post but this is what I know and what I did:
I wanted my POP accounts (personal domains, not gmail) to be pushed to my iPhone/iPod and to Mail.app. I also wanted to answer from the domains addresses when I want it.
What I did is forward all incoming mail of those accounts to @me.com, that worked but I was receiving email on both accounts (@me and domain), so in webmail (of the POP account) I setup the discard all incoming mail option. That is something that my webmail have as an option, I don't know if this can be made in gmail or other POP accounts. Discarding incoming mail and forward option make me receive only emails on my @me acoount.
What I ment before on keeping mailbox option is that my POP account it's not actually a forward account, is a real mailbox with the ability to send messages but not receiving them.
Whenever somebody writes me at my POP acoount, I receive that mail in the @me account and I'm able to answer that mail from the POP address that it was directed (not the @me account).
To be able to do this I had to setup (or synchronize) my existing POP acounts to the iPhone/iPod Touch, that gives you the possibility to "answer" whatever mail received om the @me.com account from the domain it wa originally directed to. It os not actually necessary to setup the "fetch" option inthe iPhone, since the mailbox it's not going to receive any mail.
Just to clarified on some post I've seen: I do have pushed mail in all my accounts including my own POP ones.
The only thing that can't be done it's when you compose a message in the iPhone from the POP account and have that sent mail synched to your computer. Other than that it's working perfect for me.