During a call just select the "back" button till you get to the Home screen. Then you can access anything as usual. If at any time you need to return to the "Call" Screen select the green call button.
This is how it has worked on every ROM I installed on my G1 from 1.5 to the 2.1 based ROM I currently have installed.
Galaxy Nexus & Asus Transformer (TF101)
ATT Samsung S - Captivate
Various Window OS devices
Always waiting (patiently) for the latest CyanogenMOD
Galaxy Nexus (LTE) - running AOKP ROM = Awesome!
Thanks to JesusFreke, Cyanogen, LucidRem, Stericson, Haykuro, The Dude, Wysie_Soh, Kmobs, Koush, Wes, Birdman, etc. - Ya'll are awesome!