UPDATE: June 25, 2012 – Thanks for all of the bug reports. We have enabled this feature for all VIP sites hosted on WordPress.com. For debugging, you can append ?concat_js=no to any URL to disable Javascript concatenation.
Today we are enabling Javascript concatenation on WordPress.com VIP for beta testing. Similar to the CSS concatenation we announced last week (and which is now live), JS concatenation improves performance on your sites by combining multiple JS files into a single request, which decreases the number of separate HTTP requests made on every page load.
You can test your sites by appending ?concat_js=yes to any page while you’re logged in to WordPress.com. For example, https://lobby.vip.wordpress.com/?concat_js=yes.
We will be enabling JS concatenation for all users next Monday, June 18th. Please test your site now, and if you notice any errors or issues that might be related, get in touch.
WordPress VIP Lobby 
Paul – This may be a silly question, but this feature is only for hosted sites at present, correct? If so, might this feature work it’s way into a future WordPress release at some point?
You’re correct — this is specific to sites hosted on our WordPress.com VIP platform. For self-hosted sites, I would take a look at the plugin directory on WordPress.org — there are numerous plugins (of varying complexity and feature sets) that support concatenation.
I have a question regrading compression. Before the concatenation feature (which is awesome) the files were compressed. Is there a plan to enable compression with the concatenation when code is deployed or is it up to us to compress before deploying?
Yes, the plan is to add auto-minification of the files as well.