Just seen (only a month late) WriteRoom has been updated on iOS and it looks like a good one.
I'm not sure how long there has been an iPad version/iPad version was added but I don't remember using it before. It looks great and now has Dropbox support. I have been using Elements for my writing needs on iOS for a while, mostly for it's Dropbox support, but WriteRoom seems to be doing a great job whilst I am writing this.
This version 3.0 of WriteRoom has also added an extra, customisable row of keys to the iPads keyboard. I have just started to use Markdown for my posts and this will help a lot on the iPad; it just makes getting to the square brackets fast, it also adds a tab key. The only thing that's missing for Markdown is a Markdown preview which Elements does well.