Once you are rooted, you can use adb to push a TrueType font file over to /system/fonts. (You can use Root Explorer, too)
AndroidClock.ttf, AndroidClock_Highlight.ttf and AndroidClock_Solid.ttf
I dont know if you need to replace them all, but I did, all with the same .ttf font that I chose. Once you replace those three files reboot and your notification area font and lock screen clock font will be changed.