If you are using the YNAB iPhone app, please do not update your iPhone's OS to the new OS: iOS 4. (It is available to early adopters now, and will be released officially by Apple on Monday, June 21, 2010.) This OS update breaks the YNAB iPhone app. We submitted a fixed version of the iPhone app to Apple earlier this week, but it won't be available until Apple approves it, and that could take them a few more days. (They are notoriously difficult to predict with things like this).
If you do update to iOS 4, all of your categories and transactions will be out of order. This means that some categories will be listed under the wrong master categories, making the data appear broken. (This happened because iOS 4 changes the way the phone returns sorted data to our app.)
We will post updates here in the forums, as well as on the blog, on my Twitter, and on Jesse's Twitter. Hopefully soon I'll be able to give you the good news that Apple has approved our app update! In the meantime, please resist updating your iPhone's OS!