When Tiger came out, some people noticed right away that Mail.app no longer allowed itself to be launched in a hidden state. An AppleScript fixed the problem in 10.4, but it doesn’t work in Leopard.
Martin Patfield has created a fix for Leopard users who want to launch Mail but not see it.
Installing the app is easy, and follows the same procedure as the old Tiger fix. First you need to download it, and install it somewhere on your hard drive (in the Applications folder perhaps).
Open System Preferences and navigate to the Login Items tab of the Accounts Preference pane. Select the “Plus” button underneath the list:
Navigate to wherever you saved the “Launch and Hide Mail.app” app and add it:
There’s only one problem. It doesn’t work. It launches Mail.app alright, but doesn’t hide it. At least, for me it doesn’t, although you may have better luck. I noticed that 98 people have downloaded it from MacUpdate and no one else has complained.
Try it out 
for yourself. It’s freeware (donations not refused!).
UPDATE: The developer has just sent me a tweaked copy of the app. Now it works! The entry on MacUpdate now points to the updated file.
For me it doesn’t work too!
I installed it and rebooted but’s not working!
I have been using spaces with Mail.app maximized on it’s own space. I just switch spaces and there is my email full screen, but if I want it out of the way, I switch back to desktop 1 with all of my other apps. I’ve also got a space with VMWare running full screen, and one with a maximized terminal.app.
The developer has just sent me a tweaked copy of the app. Now it works! Hopefully the entry on MacUpdate will soon be replaced with the new one.
I just put everything in a Space and hiding isn’t necessary anymore.
Thanks Tim. Works well.