Look in Console and see what the error reports are showing for iMovie. There may be clues in there. You can also try deleting the app and reinstalling it.
I have already deleted it and reinstalled it with the iLife disk. Here is the log I get, I didn't post the binaries because they are so long, but tell me if I do.
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Process: iMovie [35095]
Path: /Applications/iMovie.app/Contents/MacOS/iMovie
Identifier: com.apple.iMovieApp
Version: 9.0 (1073)
Build Info: iMovieApp-10730000~16
Code Type: X86 (Native)
Parent Process: launchd [992]
Date/Time: 2012-06-25 18:55:03.871 -0500
OS Version: Mac OS X Server 10.7.4 (11E53)
Report Version: 9
Interval Since Last Report: 98588 sec
Crashes Since Last Report: 36
Per-App Crashes Since Last Report: 5
Anonymous UUID: 2589A89A-9402-4F93-B079-339E3E107727
Crashed Thread: 0 Dispatch queue: com.apple.main-thread
Exception Type: EXC_BAD_ACCESS (SIGBUS)
Exception Codes: KERN_PROTECTION_FAILURE at 0x0000000000000034
VM Regions Near 0x34:
--> __PAGEZERO 0000000000000000-0000000000001000 [ 4K] ---/--- SM=NUL /Applications/iMovie.app/Contents/MacOS/iMovie
__TEXT 0000000000001000-0000000000412000 [ 4164K] r-x/rwx SM=COW /Applications/iMovie.app/Contents/MacOS/iMovie
Application Specific Information:
objc[35095]: garbage collection is OFF
Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0 dyld 0x8fe83063 dyld::registerAddCallback(void (*)(mach_header const*, long)) + 61
1 libdyld.dylib 0x03726078 _dyld_register_func_for_add_image + 74
2 libignitor.dylib 0x004c8e76 initializer + 27
3 dyld 0x8fe91203 ImageLoaderMachO::doModInitFunctions(ImageLoader::LinkContext const&) + 251
4 dyld 0x8fe90d68 ImageLoaderMachO::doInitialization(ImageLoader::LinkContext const&) + 64
5 dyld 0x8fe8e2c8 ImageLoader::recursiveInitialization(ImageLoader::LinkContext const&, unsigned int, ImageLoader::InitializerTimingList&) + 256
6 dyld 0x8fe8f268 ImageLoader::runInitializers(ImageLoader::LinkContext const&, ImageLoader::InitializerTimingList&) + 62
7 dyld 0x8fe83664 dyld::initializeMainExecutable() + 166
8 dyld 0x8fe87f99 dyld::_main(macho_header const*, unsigned long, int, char const**, char const**, char const**) + 2238
9 dyld 0x8fe812ef dyldbootstrap::start(macho_header const*, int, char const**, long, macho_header const*) + 637
10 dyld 0x8fe81063 _dyld_start + 51
Thread 0 crashed with X86 Thread State (32-bit):
eax: 0x8fe83034 ebx: 0x0c46e000 ecx: 0x004c8e7c edx: 0x00000000
edi: 0x8feb6dac esi: 0x00000000 ebp: 0xbfffe9a8 esp: 0xbfffe980
ss: 0x00000023 efl: 0x00010286 eip: 0x8fe83063 cs: 0x0000001b
ds: 0x00000023 es: 0x00000023 fs: 0x00000000 gs: 0x0000000f
cr2: 0x00000034
Logical CPU: 1