df -k|perl -ape'map{while(1024<$_){$_/=1024;$i++}$i="%6.1f".substr"KMG
+",$i,1;$_=sprintf$i,$_}@F[1..3];$_=join"\t",@F,$/'
[download]

Output using the sample data looks like this:

/dev/dsk/c0t1d0s1          1.2G  795.7M  382.2M 68%     /usr
/dev/dsk/c0t1d0s3        239.7M   43.5M  172.2M 21%     /var
/dev/dsk/c0t1d0s0        414.6M  182.8M  190.3M 49%     /home
[download]

I didn't come up with a way to cleanly format G to 1 decimal place, leaving K and M at a whole number (ie 1.2G, 240M).

94 if less precision and un-aligned formatting meets your standards :-P

df -k|perl -ape'map{while(1024<$_){$_>>=10;my$i++}$_.=substr"KMG",$i,1
+}@F[1..3];$_=join"\t",@F,$/'


Output:
/dev/dsk/c0t1d0s1       1G      795M    382M    68%     /usr
/dev/dsk/c0t1d0s3       239M    43M     172M    21%     /var
/dev/dsk/c0t1d0s0       414M    182M    190M    49%     /home
[download]

Update: fixed miscounts