When I install AIOSEO plugin in sites usign wp 2.9.2, it causes something like this:
to appear at the top of pages.
and of course, causes the pages to be invalid with respect to testing at w3, which suggests that perhaps a quote was left unclose (previously to that insert in the wp_head() area of the page) or that entities were/are not being handled properly (ampersands and html/php brackets)
This is the second time it's happened for me in two completely different sites (custom)... not sure what's going on and I did try very hard to fix it, but NO JOY.
Wish it were fixed so I could recommend this seo plugin to clients.