It seems you have to do the following on a PC. I've tried doing to with astro, root explorer, etc on the phone and the changes don't seem to take for some reason, but if I pull the sd card from the phone, insert it into my handy dandy usb adapter and plug it into my PC and make the following changes it works every time...and I currently have every posted theme modded with the iPhone sms/mms theme.
If you open the iphone.zip sms theme you'll see inside is the folder com.android.mms. pull/copy that out of the zip so its on your hard drive. Now open any of the theme.zip files. Many of them don't even contain a com.android.mms folder. If not, simply drag the com.android.mms folder from your hard drive and add it the theme.zip of your choice and click save or save as. If you want to add it to a theme with a pre-existing com.android.mms folder, delete that folder that you already see in the zip, then drag and add the iPhone com.android.mms folder to that zip, and again click save.
Done deal...