- If you're using JS for progressive enhancement on an otherwise static site, e.g. form validation, pjax, image carousels, tabs, animations... simply drop that JS for 6-8. Form validation should just round-trip to the server as it has to do anyway, pages should just link normally, other visual enhancements can be dropped. That's the progressive enhancement way after all.
- If you're trying to build a single-page web app for IE 6-8 and this is your reason for wanting jQuery to work, then you're going to find those browsers have JS engines that are incredibly slow and a JS-heavy web app for oldIE is not going to deliver a good user experience even if you get it to 'work'. I think frankly this is an unrealistic goal from the get-go. The pitiful speed of JS on these browsers is something that does seem to be often overlooked.
If you absolutely have to push a single-page web app to a corporate customer stuck on oldIE, Chrome Frame is a very sensible option. This has served us well and we've yet to encounter a customer who complains, as long as you give them plenty of forewarning and explanation. It can be tied down to only trigger on your site, so sysadmins really have nothing to gripe about with respect to FUD about attack vectors etc.
Anyway I'm not saying 'you definitely shouldn't be serving JS to oldIE' I'm just putting it out there as something to consider, as usual in this business there's no single answer.