to write a program (in any language)

Would it offend the dear monks if I posted a program in J then? I hope no. (Even though you say any language, that's for the original question, ant this is perlmonks.)

Here's the program:

spiral =: 3 : 0
>: (,~ y.) $ (i. *: y.) i.~ /: ,/ (-@>.&| , -@{:@*.@(1j_1&*)@j.)"0/~ (
+i. y.) - -:<: y.
)
2!:55[0[ 1!:2&2 spiral 5".>{: ARGV
[download]

To run it,

1. first get a J User Licence by filling this form;
2. download J;
3. set it up by the instructions in readme.txt (comes with the download);
4. save the above code as spiral.ijs (don't break the long line);
5. and run jconsole spiral.ijs 5 (5 is an integer giving the spiral size, defaults to 5).

(You can of course skip the first four steps if you have already done them.)

Update 2007 dec 7: better definition for the spiral function is

spiral =: - ]\ i.@:*: [`]`[} [: \: [: ,/ [: (>.&| , 12&o.@(1j_1&*)@j.)
+"0/~ i. - -:@:<:
[download]