Agree... you must visit m.google.com/sync to register your device settings.
As for Lion/Apple Mail... I'm no pro... but this is what I did and my alias is working like a charm...
1. System Preferences >
2. Mail, Contacts, & Calendars >
3. Click on + Add Account and select Other.
4. Choose the first radio button, Add a Mail account > Create.
5. Place your alias email address in the email field and enter your password.
You will get a message saying Mail couldn't log into the Mail server "gmail.com." Just hit Continue anyway.
6. Select IMAP for Account Type.
7. Replace the User Name (was your alias) with your entire actual/primary gmail address > Continue.
8. Place smtp.YourAliasDomain.com in the Outgoind Mail Server field.
9. Check Use Authentication > and enter your entire alias email address in the User Name Field > Continue.
Apple Mail should create the account... you may need to "take the account online."
Hope this helps.