That's why wrote this AppleScript, which does those three things automatically. Note that you can also find this script here on LibSyn.
It requires installation of the LAME binary from their downloads page, which in turn requires Xcode, which can be installed from your OS X install disc, or downloaded from Apple's Developer Connection website (free account required). It also requires that a few paths be customized within the AppleScript.
When the script is run, it asks you for an input file, which must be lossless -- AIFF or WAV would work. (It is important to do a dry run to make sure that your files will encode properly. Audio Hijack Pro, for example, produces AIFC files with .aiff extensions which do not encode properly.) The script then asks you for an output file name and an episode name. It then creates an MP3-encoded file called deleteme.mp3 in a directory that you specify, imports it into iTunes, adds ID3 tags (including, optionally, a photo), and uses curl to upload it to LibSyn. If you have an account elsewhere, obviously, the command can be changed accordingly. Finally, it deletes deleteme.mp3.
When it's done, two copies have been generated -- the one in your iTunes library and the one on the remote server.
It's fully documented, and it turns three tasks that require your attention into one big task that doesn't -- a big improvement. The one thing I wasn't able to figure out was how to make progress bars for the curl command... but I'm sure someone out there knows.

