gistfile1.txt

- (void)scan {
	NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
	ALAssetsGroupType sources = ALAssetsGroupLibrary | ALAssetsGroupSavedPhotos;
	NSMutableArray *tags = [[NSMutableArray alloc] init];
	
	ALAssetsGroupEnumerationResultsBlock assetsEnumerator = ^(ALAsset *result, NSUInteger index, BOOL *stop){
		NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
		
		if (result) {
			if ([[result valueForProperty:ALAssetPropertyType] isEqual:ALAssetTypePhoto]) {
				NSDictionary *metadata = [[result defaultRepresentation] metadata];
				NSDictionary *iptc = [metadata objectForKey:(id)kCGImagePropertyIPTCDictionary];
				NSArray *keys = [iptc objectForKey:(id)kCGImagePropertyIPTCKeywords];
				NSURL *path = [[result defaultRepresentation] url];

				if (keys) {
					for (id key in keys)
						if (![tags containsObject:key]) {
							[tags addObject:key];
							[keywords addObject:[[[KeywordInfoModel alloc] initWithSymbol:key andRelatedKeys:keys] autorelease]];
						}
					
					if (![paths containsObject:path]) {
						[paths addObject:path];
						[photos addObject:[[[PhotoInfoModel alloc] initWithURL:path andKeywords:keys] autorelease]];
					}
				}
			}
		}
		
		[pool release];
	};
	
	ALAssetsLibraryGroupsEnumerationResultsBlock groupEnumerator = ^(ALAssetsGroup *group, BOOL *stop){
		if (group)
			[group enumerateAssetsUsingBlock:assetsEnumerator];
		else
			[self doAfterScanning];
	};
	
	ALAssetsLibraryAccessFailureBlock errorBlock = ^(NSError *error){
		NSLog(@"Failure");
	};
	
	[self performSelectorOnMainThread:@selector(doBeforeScanning) withObject:nil waitUntilDone:YES];
	[library enumerateGroupsWithTypes:sources usingBlock:groupEnumerator failureBlock:errorBlock];
	
	[pool release];
}

You can run [group enumerateAssetsUsingBlock:assetsEnumerator] on another thread. I think that's where you’re really blocking, because [library enumerateGroupsWithTypes:sources usingBlock:groupEnumerator failureBlock:errorBlock] is done async for me. You also seem to have an unused NSAutoreleasePool.