I spent hours upon hours trying to find, install, configure, use, and uninstall several different spell checking plugins for wordpress. Some were AJAX. Some were no longer under development. All were garbage.
I've finally found the perfect solution to my problem, and that is simply: the Google Toolbar.
The Google Toolbar has an inbuilt form spell checker (the green check mark with super-scripted 'ABC' above it). It works fast, it's in-line (doesn't require a page reload), and I don't have to install and update any WordPress plugins. So, if you catch any spelling errors in my blog, it's simply because I forgot to use the checker.