mkdir pi
mv Downloads/debian6-19-04-2012.zip pi
cd pi
unzip debian6-19-04-2012.zip
umount /dev/sdb1 # or whatever your SD card mounted as when you inserted it
dd bs=1M if=debian6-19-04-2012/debian6-19-04-2012.img of=/dev/sdb
1859+1 records in
1859+1 records out
1950000000 bytes (2.0 GB) copied, 163.485 s, 11.9 MB/s