The enable_jit.zip contains the files from the Desire Thread/Unrevoked JIT Wiki and the disable_jit.zip contains the files from the Stock rom.
BE SURE TO DO A NANDROID BACKUP FIRST!!
Tested and working on:
Jager Senseless v1.7
Tested and NOT working on:
Stock (Random reboots)
CM5 Test 3 (Everything FCs on boot)
Most likely any other ROM with Sense
To see if JIT is enabled, while booted do the following:
adb shell cd /system/bin/ getprop dalvik.vm.execution-mode
All credit goes to gr0gmint who got it working it on the Desire.
Again, do a NANDROID backup before you load these and please post your results with what ROM you are running.