Depending on the font, there are limitations on web use.
There is also Cufon, or just the plain @font-face format, which is supported by most modern browsers.
You are often limited by what you can do with CSS (More so with older browsers), and with the display of the font/CSS per browser, which generally isn't the same across browsers and operating systems.
It may be possible we are talking about multiple things here. I am talking about replacing the logo
with <span class="logo">xenForo<span class="logo_top">TM </span></span> where .logo is defining the font. I am unsure how this is possible with 'extensions', if yes, than I am also very interested in that! There are lots of new things browsers can do now, and thanks to the auto-update my users browsers are mostly up to date
Just to avoid confusion; this is what I am talking about.
css3 @font-face rule support: http://www.w3schools.com/css3/css3_fonts.asp
Firefox, Chrome, Safari, and Opera support fonts of type .ttf (True Type Fonts) and .otf (OpenType Fonts).
Internet Explorer 9+ supports the new @font-face rule, but it only supports fonts of type .eot (Embedded OpenType).
Note: Internet Explorer 8 and earlier versions, do not support the new @font-face rule.
.woff is not even used by more than 0.02 % and is basically ttf, but I do hope that within a short period of time, and before html6 / css4 that it has become the standard to replace openType, ttf, and what not.
I understand the desire for a single standard, I just don't think there's nothing wrong with using what's already out there, be it TypeKit, openType.js or Google web fonts. Or just define your own yourself.
The gamma, dpi, and font render engine on various operating systems display a font completely different. A logo has to be very precise, it's usually the branding of a company. You 'can' use the coca cola font.woff, but it will be very thin and feel compressed on my old Mac Pro or even without proper css definitions un-aliased and jittery-ish on Windows. Whereas it looks almost 100% the same on as the real logo on my iMac.
You'd also have to do some trickery in regards to most logos, such as vBulletin uses Industria or whatever it was, but the logo is actually stretched vertically, and longer end-points, to be more unique. And XenForo has a strike through on some letters, etc. Most companies might end up having to create their own .woff font. Their efforts are perhaps better spent on .svg-ing their logo.