booting ubuntu from a pendrive

please note that ideally the size of the disk should not be more than 2 gb though this may not be a hard and fast rule.
in hardy and later these instructions apply. in terminal window in ubuntu type
$sudo apt-get install syslinux mtools
#the $ sign represents the linux prompt.
now find out the device address of your flash drive with fdisk -l command.
assuming your flash drive is /dev/sdb1, the command for making the drive bootable will be as follows
$sudo syslinux -s /dev/sdb1
next copy the contents of your ubuntu cd to the disk including the hidden items.
Now find the isolinux folder and rename it to syslinux. go inside this folder ad find the file isolinux.cfg and rename it to syslinux.cfg and you're done.
you will be amazed at the speed with which this pendrive works. you can use it to install as well as use it like the live cd.