Try keychain scripting or using terminal command "Security"

set theUserName to do shell script ("security  find-generic-password -gl ****KEYCHAIN ENTRY NAME*** | grep \"acct\" | cut -c 19-99 | sed 's/\"//g'")

set thePassword to do shell script ("security 2>&1 >/dev/null find-generic-password -gl ****KEYCHAIN ENTRY NAME*** | cut -c 11-99 | sed 's/\"//g'")

tell application "Keychain Scripting"
	set myKey to first key of current keychain whose name is "****KEYCHAIN ENTRY NAME***"
	set theUserName to account of myKey
	set thePassword to password of myKey
end tell

I should note that you'll have to setup, in keychain, a new Generic key with the password that you need to use. Then, it should be named *** KEYCHAIN ENTRY NAME ****

Thanks for the code, Andrew!

I have packaged it up in a nice little app that allows you to start and stop MAMP's Apache and MySQL servers with a single click and doesn't ask for a password each time. It has a nice icon as well :)

You can download it from my blog:
http://www.46palermo.com/blog/run-mamp-without-password-easy-way/

Regards,
Damian