You can do this, on a Mac, with Quicksilver and either email or Twitter. Lifehacker wrote about a Windoze Quicksilver-like app recently, so if you're not on a Mac, you might try that, too.

http://www.rememberthemilk.com/forums/ideas/2410/ is one thread on it.

Excellent, thanks for those. I also came across Launchy, which is similar to Quicksilver but for Windows. Check this out:
http://lifehacker.com/software/hack-attack/take-launchy-beyond-application-launching-284127.php

no longer works with launchy. :-/

I wonder how many people have tried doing quick add from a twitter desktop app. Seems like it would be convenient enough.

The best alternative that I have come up with is to just dedicate a browser to RTM. Basically, Chrome is my primary browser and I run Firefox with the A Bit Better RTM extension as a dedicated RTM app.

The technically-inclined may ask: Why not use Fluid or a site-specific browser? Because add-on/extension support is a bit flaky in Fluid and A Bit Better RTM is essential. Especially since the good folks at RTM have no interest in building a keyboard shortcut to move a task between lists.

If RTM wasn't so stingy with their API keys, I would have developed something for this already... hint hint RTM :)

Why has a feature like this not been implemented for nine years?