We've been quite surprised to learn that a lot of people are using proportional-width fonts for code and markup, but apparently that's the case. Unfortunately, TextMate is in love with fixed-width fonts. And that affair is unlikely to change any time soon, so we recommend taking a look at either Monaco, Courier, or ProFont (be sure to use ProFontIsoLatin1). The latter is what we used in the screenshots and videos on the main site. It's free and mighty good looking.
P.S.: You get a much better and cleaner look for ProFont by setting "System Preferences > Appearances > Font smoothing style" to light. Remember to restart TextMate afterwards.