From https://github.com/phonegap/phonegap-iphone/issues/148:
NSURLCache* cache = [NSURLCache sharedURLCache];
[cache setMemoryCapacity:4 * 1024 * 1024];
[cache setDiskCapacity:512*1024];
[NSURLRequest requestWithURL:appURL
cachePolicy:NSURLRequestReturnCacheDataElseLoad
timeoutInterval:10.0];