There appears to be several.
There is
Markup. The UI is nice but the reviews are terrible. Also no syntax highlighting in the screenshots.
Then there is
HTML Edit. Looking at the screenshots, the interface is rather bad. Some okay review though.
Gusto. Best interface of the 3, with the exception of syntax highlighting. Some pretty good review too.
Just 3 I could find and I am sure there are more or others built in to other apps. Gusto is not in UK store though, which is a shame for me. But since I don't even have my iPad yet I am sure it will be out eventually.
The only problem I see is using the iPad keyboard to do all this. I guess there is the bluetooth one though.