So are you Fred
There is also some useful photos showing how the complexity of the Space Shuttle External Tank at - http://www.nasa.gov/centers/marshall...e/shuttle.html
The major problem for the foam is the PAL area and Bipod region. The Bipod area creates significant turbulence - vortex shedding off the cylindrical structures.
It appears that the area on ET-121 (on STS-114, Discovery), which lost the foam, was a patch and blend repair. There is definitely a need for improvement in this area.
In reality, the foam loss is only significant on the shuttle side of the tank or roughly 1/3 of the circumferential surface. A kevlar mesh with about 5-8 cm pitch and located just below the surface of the foam would seem reasonable to ensure a reduction in foam loss - but perhaps easier said than done.