When you do the offscreen webview thing, you need to call

[_webView setHostWindow:someWindow] on your webview. That is in the docs
what I found was that I got your exact problem until I added the webview as a subview of the window:
[[someWindow contentView] addSubview:_webView];
[_webView setHostWindow:someWindow];

Then in dealloc, remember to release everything (i was using a shared stub window so I only call removeFromSuperView.
dealloc –

[[NSNotificationCenter defaultCenter] removeObserver:self];

[_webView setFrameLoadDelegate:nil];

// remove the subview from the host window:
[_webView removeFromSuperview];
[_webView setHostWindow:nil];
[_webView release];

Also like the seperate task idea. My problem was that I am already in a ‘helper’ task!

–Tom