now is there was a way to "create textmate document here" - like an explorer shell extension in windows... when browsing a folder in the finder. (maybe this is a quicksilver thing?)
The easiest way to do this with vanilla os x is to open a new document in textmate, hit cmd-s, drag the folder you want to save this in into the save box (which will then change into that directory), enter a filename, hit enter. You can drag the 'proxy icon' of the directory as well (proxy icon = the icon in the title bar).
The easiest way to add a real 'create textmate document here' is probably with OnMyCommand, but there are other easy ways for sure. I have an 'open terminal here' context menu entry which I use with 'mate filename' to open new textmate files where I want. I need the terminal anyways most of the time, so this is more useful for me.
HTH.