I'm not sure how or why, but this has now happened twice in a row. The first time I thought it was a flue when it disappeared when upgrading from 2.4.5 to 2.4.6. I went to check the settings and noticed it wasn't listed, then checked installed plugins, and nada -- WP to Twitter was absent. The plugin dir itself still existed, but was empty, except for the js dir. I manually uploaded 2.4.6 and didn't give it a second though.
Today, I did the automatic upgrade to 2.4.7, and the same thing happened -- WP to Twitter was gone again, this time only leaving the js with "jquery.charcount.js" inside. Regrettably, I didn't pay close attention to the post-upgrade output to see if it noted anything, however there's nothing unusual in my setup compared to the last several times I (successfully) updated WP to Twitter.
If it helps you, I'll downgrade to 2.4.6 and re-attempt upgrade and this time note the output (assuming there's no major risk in doing so).
Err, scratch that idea -- I already overwrote my local zip copy of 2.4.6 with the new 2.4.7, so I no longer have a copy of 2.4.6 to downgrade to.