To install an archive with a rootfs (Debian or Angstrom) on SD card, please follow these steps:
Insert an SD card into a cardreader on a Linux based development system. It will show up as /dev/sdb1 or /dev/sdc1 or similar. We will use /dev/sdb1 and assume that /mnt/card and is an empty directory, otherwise create it. If it gets automounted (the command mount will give you a list of mounted devices and their mount point), first umount it. You need to have root rights to do the following steps:

umount /dev/sdb1

Then format the SD card with ext3:

mkfs.ext3 /dev/sdb1

Now it should be mounted.:

mount /dev/sdb1 /mnt/card

Unpack the archive to your card:

tar -xvf taskit-rootfs.tar -C /mnt/card

The name of the archive may differ. If you encounter a .bz2 use tar -xjvf, if a .gz use tar -xzvf.
Finally umount the card:

umount /dev/sdb1

Wait until the command returned, before ejecting the card. Now the card can be used on your target system.