It's working on my end with the following simple example:
[self.webview loadRequest:[NSURLRequest requestWithURL:[NSURL URLWithString:@"http://mobile.twitter.com"]]];
If you are caching cookies manually (using NSCookieManager, etc., that could be the cause.
Few questions to help debug this:
Are you seeing this inside of a UIWebView, or a custom view?
What is the exact URL that you're pointing to?
What iOS SDK?