Before i go ahead and buy one of these, i have a couple of questions.
1. It comes with ubuntu 9.04, therefore does canonical host repositories with ARM software? If so will apt-get update and upgrade work as if i were running standard x86 ubuntu?
Yes, that is correct. However the ARM package set is smaller than x86, but most stuff is there.
2. When karmic comes out, can i update to that as easy as i would do on x86?
No, for reasons only known to Canonical, they are dropping ARMV5 support in karmic, so no upgrade path for Sheevaplug. However debian is continuing to maintain packages. So a switch to Debian might be in order (or Gentoo).
3. My main requirements for this is to be able to download torrents, setup ftp, samba and printing. I have no real experience using ubuntu in this way other than torrents. How easy is this to achieve? I want to use ktorrent with its web interface for downloading torrents. What do you recommend for the others?
I do most of that stuff on my plug, and it's reasonably straightforward. I use rTorrent (and wTorrent, it's web interface). I use mine as a media server, SMB, HTTP server.
4. My only other requirement is that my 500Gig external usb hdd will work with it. I assume it will, but will i need to edit fstab? or will it just work?
Should do, there is a problem with the speed the plug boots up at, (it boots too fast, and sometimes USB HDD's have not spun up), there is a script to pause and wait for disks to spin up on the forums. So, yet it will work, but might require a little fiddling.