Besides X-Plane for iPhone (which I now call "X-Plane general aviation" to avoid confusion) there are now two new apps: X-Plane Airliner and X-Plane Helicopter. The helicopter version uses part of the Grand Canyon and the airliner version uses part of Southern California.
All three apps (the general aviation version has a free update) have a fix in the DSF lower that should help avoid crashes.
Basically while X-Plane used to run under memory limits for the phone, it would temporarily go quite a bit over memory the limit during the DSF load, as the DSF loader would use some temporary memory. The new code very carefully purges temporary memory as it runs, and thus never exceeds its final memory footprint. Before 9.04 there was always a risk that your phone was in a tight memory situation to begin with, such that X-Plane going "over budget" would cause the OS to kill it off. (Rebooting the phone apparently purges memory or something.)
So...this is a long-winded way of saying: if you update X-Plane iPhone to 9.04 and still have the app quit at launch (or right after launch), please send us a crash report!