Wonder how it interacts with browser cacheing and parallel JS resource downloads?
This will have very little effect on caching. You will still need to configure your web server for optimal caching, by setting the Cache-control and Expires headers, for example.
Even so, this still has two main benefits: The JavaScript is minified, which will reduce the amount of raw data sent to remote users. Plus, by concatenating all the files together, you avoid the need for multiple separate HTTP requests.
AFAIK, that includes the latest safari, firefox, and IE8.
However, it's still obviously a good idea to concat and minify all your scripts.
I had previously been told that a JavaScript download basically halts the entire processing of the remainder of the page -- including any downloads that might occur there (CSS, images, and more JS) -- because the browser can't start parsing the rest of the page until it has interpreted the JavaScript. That's why it's best to move your JavaScript down to the bottom of the page.
Is this just the way older browsers used to behave? Are there still any restrictions like this that I should be aware of?
eg gzip(concat(A, B)) compresses to a smaller file than gzip(A) + gzip(B)
For example, if I take 20 js files from Mibbit and gzip them individually, total size is 47,890 If I concat then gzip, it's 42,681