Well there are a lot of great themes for Summerboard, but also a lot that are .ipb. Is there anyway to convert .ipb themes to Summerboard? Its a lot easier IMO to have summerboard themes opposed to .ipb themes.
The easiest way ( ) might be to install the ipb theme and then use iPhoneBrowser to save your current setup as a SummerBoard theme. Then you can copy your SummerBoard theme folder back onto the phone.
To do it manually, you will have to unzip the ipb and then manually copy and rename each applications icon.png to the corresponding SummerBoard appname.png and put it in a folder.
And the SummerBoard theme will not have custom Carrier logos, sliders, etc.