Twitter has just released a fairly substantial update to its iOS applications. There are some nice new features here: a cleaner interface for composing your Tweets, an easier tool for uploading photos, auto-shortening for links (to the official t.co shortener), and autocomplete for usernames and hashtags.

It also appears to include a fix to one of the things I found most irritating with the older version of the iPhone app: that old, read Direct Messages perennially appeared as unread.

But there's an addition to the app that Twitter describes as "very cool" that I find much less so. It's a Quick Bar, and it appears at the top of your timeline. The Quick Bar highlights what's currently trending on Twitter, and you can swipe the bar in order to see additional topics.

The trends right now include (no surprises here) #winning, Charlie Sheen, and #tigerblood. Also trending, Google Hotpot. That's a promoted trend today, coinciding with Google's announcement today that you can Tweet your Hotpot reviews from within the Google Maps app on Android.

Although the Quick Bar is no more intrusive than a lot of ads you'll find in mobile apps, I'll admit, it's annoying. It hovers at the top of your timeline, no matter where you scroll. You can't turn it off. (You can, however, turn off local trends, another new feature that uses your location to deliver you information about what's popular in your vicinity.)

For users of the Twitter app (and the Tweetie app before it), it may be the first time they've seen ads in their Twitter streams. Some other (unofficial) Twitter apps have long been ad-supported, and it feels like the new Quick Bar feature is a portent of more advertising to come.

The Quick Bar is only on the iPhone version of the app. For now.