Totally thought of Yoni when I saw this. Would have been better to make it pi [smallfont]r[/sf]^2 to make it more hl2ish but still mathish too. :P

Eh. Could be cooler. Pi != 3.14.

Would be rather hard to fit pi into that little spot.

I'm sure something like 4*sum[n=1,infinity]((-1)^(n+1)/(2n-1)) could be written in a small enough font...

Edit: In the spirit of this forum I'll add a link.
http://mathworld.wolfram.com/PiFormulas.html

[color=Pink]That's a awesome wallpapre, i set it as mine~[/color]

Only now found this.
Not a wallpaper, but still uber awesome.
http://www.thinkgeek.com/tshirts/science/6e7e/zoom/
Buy me one.

Sweet shirt!! I want one as well :D