BikeForums is on the very latest version of vBulletin (release 4.1.12). The auto save feature has been available since (IIRC) version 4.1.4 or so, but it didn't announce itself as it does now.
Here's a blurb from the software vendor about Auto Save:
Content auto-save is like an emergency parachute – it automatically saves your content so that it can be restored in the event that things go very wrong.
Imagine this scenario. You’ve sat down to write a blog entry or a really long reply to a thread. Like a mad composer, you are banging away on the keyboard typing away furiously; your words and ideas are flowing freely into the editor. In an unfortunate twist of cruel fate your foot kicks the power outlet and unplugs your computer. Your computer shuts down as you begin to panic – the panic slowly transforming into agonizing rage. Well, there is good news! The new CKEditor automatically backs up your work every 30 seconds (this is adjustable). You curse the skies, successfully reboot your computer and navigate back to the page where you were writing the content, click the “Restore Auto-Save” button and magically – your masterpiece is now restored. Nothing has been lost!
This magic auto-save functionality applies to the editor in general and is available in all vBulletin sections, including CMS, Forums and Blogs.
Try this:
Start typing a reply.
Stop - go and do something else for a minute or two.
Kill your browser. Just close it.
Re-open your browser, and navigate back to the exact page you were on.
There's now a small button on the bottom-left of your text box, labeled "Restore Auto-Save”. Click it.
Bingo - the text you'd started typing, before you cruelly killed your browser, is back.