According to Installer, Twinkle from Gogo Apps is "Twitter plus Location." It's a Twitter client that adds photos and geolocation as well as clickable URLs and Follow / Stop Following control within the app. With it, you can find people twittering near you -- in fact it seems to be using my findme utility to power its location-awareness. And no, I have no financial interest in this thing -- I gave permission for them to use the utility so long as the app remains non-commercial.
I logged in, entered my Twitter account info (yes, the account information is stored in clear text in ~mobile/Library/Preferences) and within seconds, it found tweets within 50 miles of my location.
You can tweet directly from Twinkle. Tap the pencil button at the top-right corner (it's the one that for quite a while I mistook for a mis-shaped lower-case "i"). From there, you can enter your tweet, specify whether to tag with a location and optionally snap a photo.
Twinkle offers a professionally designed interface and good ease of use. The only negative occurred after I mistyped my password. I ended up in an infinite loop of authentication errors and had to force-quit the application.