Hello, folks. Pardon the length of this email, but I wanted to throw my hat into this ring.
I've just downloaded TextMate, and it has taken much restraint to suppress the constant urge to toss that ugly icon into the Trash. I'll make no apologies for the fact that I find it hideous, and if it were up to me, I'd start over from scratch with a completely new design. Alas, I am not a designer, so I cannot give you a mockup. But I would like to get a description of my idea out into the open, so that someone with artistic skills could give it a go.
A Mac OS X application icon is supposed to show the type of document the program works with, and a tool that one might use to work with that kind of document. See TextEdit, AppleWorks, and Preview. However since there are many apps that do not exactly follow this guideline -- I'm thinking of iTunes, Safari, Mail, and Address Book -- I think we have a little room for interpretation. But I am still in favor of keeping the same visual style and perspective used in the Apple icons.
I envision the finished icon as mostly white. The "document" part of my icon idea is two sheets of white unlined paper, at the familiar angle (see TextEdit), which look like they could be homework assignments or test papers. I envision that text is hand-written on the papers -- but it should be made clear from the indentation and varying line length and such that it is programming code, not paragraphs of text. There should be red markings on it, indicating where corrections have been made, and possibly a red hand-written and circled A at the top of the page, indicating the grade this assignment has theoretically received, or just a checkmark or some other indication that the document is now A-OK, thanks to TextMate.
The "tool" part is where I think we can depart from the recommendations a bit, given the state of other Apple icons. I can't get along with the current robot, so I suggest a happy anime cartoon boy. In anime, as in other cartoons, characters often have superpowers or hidden abilities, and this guy's power therefore is sprucing up your document quickly and efficiently without getting in your way. He's standing proudly to the right side of the papers, legs slightly apart, arms folded in front of him, a red marker clearly visible in his hand, and with a stereotypical spiky outrageously-colored anime hairstyle, possibly partially hidden by a backwards-turned baseball cap. I see him wearing long white pants and a white shirt or sweatshirt (and if only MacroMates had a logo, it could be printed on the sweatshirt). I see the hair as being the icon's primary source of color, but this could prove to be too little color, so the shirt and/or pants may have to get some color too. The color of the SubEthaEdit icon changed from blue to green with the 2.0 release, and iTunes' icon has changed color with every major version as well; the hair and/or clothing colors of this icon could also be changed across TextMate versions if desired.
Document icons should be easy to make based on this idea -- just the marked-up paper by itself, not at an angle, with the customary turned page corner.
So that's the idea. If we have any designers out there wanting to tackle it, please go for it! I'm of course available by email for further discussions of this idea, and I'll try to monitor the list as well.