The issue is that SMF2 does support IE6 nearly perfectly, and the little man inside me keeps saying, "if they do, why don't you!"... But OTOH another little man says "that's precisely because they support it that they can't move forward in terms of design"...
So, long ago I chose the mid-way solution: make sure it's usable in IE6, but screw any concepts of 'making it look the same as other browsers'... The problem is that IE7 is also quite hard to keep in line. Not as hard as IE6, but still annoying. And IE6+IE7 = not a negligible market share.
Thankfully, most of the IE6 users are in China (about a third of the users over there...), so if you don't plan to have a Chinese version of your website or to appeal to Chinese users in general, then you don't need to bother with IE6 at all.
As for vertical alignment, my main problem really is that even for modern browsers, providing flawless alignment requires at least one extra div nesting (a display:table then a series of display:table-cell). I just... Don't like that. Also, because it uses a table layout, I would need to change the order in which cells are shown (with a float layout, it's: left-right-center, while a table layout has a more regular left-center-right.)
So basically, let's just say that I made a lot of fixes to that six months ago, and I'm not in a hurry to rewrite these fixes just to make it all work in a table layout.
In the end, I decided against going inline-block or table, and staying with my current solution. After all, it works... It just doesn't work *as well* when the header takes at least two rows. But OTOH, this would never happen in a normal situation -- only place where it COULD happen is the current posthead (a custom header that shows the topic title in large characters, surrounded by previous/next topic links in plain view). And guess what? From the beginning this one had a table layout anyway...

Ah, you have no idea how lonely it is to get into vertical alignments...

I don't know if it's related, but the resulting commit will be revision.... 666.