From what i've seen in iPhone applications, an application should only have one tab bar view that lets you select its major feature areas. Is it confusing or against HIG to do this for example:
tap a tab, a table view gets displayed, select a cell, a new tab bar view gets loaded.
It is confusing and specifically recommended against in Apples HIGs. They are okay with tab controllers having multiple navigation controllers, but not vice versa.
Also it would be confusing for the user as most iphone apps have a stable set of tabs (even if they are sometimes hidden or replaced with toolbars.
good luck.
carl