You can approximate Skitch with the Quicksilver “Screen Capture” and “Upload to Flickr…” actions. Just capture part of the screen and choose the Run action. (More details in this Lifehacker article.) Then QS will capture the image and open it up to operate on. Then hit tab and type begin typing “Flickr” or “Upload” to get the upload action. If you want to tag it too, hit tab again and type them in. Then just hit return to send the picture up to Flickr.
I haven’t used Skitch yet, but this is what I understand it does. At least, that’s what I wanted it for. It’s in closed beta right now.