After researching this problem for days... I think I've finally found an acceptable solution.

Ubercart: 6.x-2.x-dev, probably works fine with 6.x-2.4
Ubercart Ajax Cart: No! Isn't it kind of funky anyway?
Boost: 6.x-1.18

So basically Boost gives us the option to not cache certain pages. While you may want the Shopping Cart block on every page, it probably isn't necessary. I set the standard shopping cart block to hide when empty, and to only show on my "main" pages, and not on deeply nested/hidden pages like blog posts that anonymous first time customers are not likely to look at after adding a product. But then again, I'm not working on a standard online store so I don't know. More later.

So then you set boost to not cache these same pages (pages that your shopping cart block will appear on.)

Results:
-Even pages that have Boost page caching disabled (cart block pages) are incredibly fast. 4x faster for my slower localhost setup
-Anonymous shopping cart blocks show up as expected- hidden when empty, correct quantities with items added to the cart
-Tested with 2 anonymous users "at the same time", no overlap in items, cart ghosting, or failure to update quantities (as expected)

So I'm pretty jazzed on this and hopefully everything will hold up after further testing. Need the cart block on every page? Write a snippet to detect if the cart block is present under Boost's "Statically cache specific pages:" setting or try "Cache only the listed pages." and put nothing in the box (technically no page caching). I'm figuring that Boost's other performance gains will offset the fact that caching of pages with the Cart Block is disabled. Just theory of course. I suppose Ajax Cart is the other option, though I did not like it.

Hoping the issue will come to a close with D7. This issue is critical to usability and affects my ability to recommend Drupal+Ubercart as a viable solution to online shopping.

@joethejoe try what I'm suggesting and let me know!