How to enable the iOS spell checker in Mobile Safari?

The iOS spell checker works for me in all the iOS applications, but it doesn't seem to work for text I type in a text area in Mobile Safari. For instance, spelling errors are underlined in red for emails I type in Mail, but they are not underlined in red in the web-based Gmail. I am not talking about completion feature, but the red underline of misspelled words, which is new with iOS 4, as shown in this screenshot:

How can I get spell checking to work for form fields in Mobile Safari like it works in other applications?

It looks like it's indeed a hole in iphone's safari. I'm not sure if it was always like that or if it's a new undocumented feature that might soon go away.

But meanwhile you can try using Inspell if you're willing to jailbreak. There's just no other way to fix such an issue: it's either apple or cydia.

Another option, if the error occured only in safari, is using another browser. But that doesn't seem to be this case.