Here's one to get people started. It weighs in at 164 162 characters and is probably disqualified for not pretty-printing the output, but it does what you ask:

df -k|perl -ne'print join"\t",map{/^\d+$/?($_=$_/1024)<1?p($_*1024)."K
+":($_=$_/1024)<1?p($_*1024)."M":p($_)."G":$_}split/\s+/;print"\n";sub
+ p{sprintf"%.2f",shift}'
[download]

CU
Robartes-

Update: Killed two spaces.