OK
rtfmoz, I didn't think to start a thread here, but here's my experience to add to the crowd:
A number of people have quoted and used the Engadget article on upgrading theTivo's hard disk... (the reference is here:
http://www.engadgethd.com/2008/04/17/how-to-upgrade-your-tivo-hd-with-winmfs ).
I'm just chiming in that I've joined the ranks of the Australian 1 TB Tivo-ers.
I had to do a couple of things that may not be for the faint-hearted:
1. I needed to get another SATA disk connection on my PC, since Tivo's original HD is SATA, the replacement has to be SATA, and the hard disk on my PC was SATA as well. Most motherboards have just two SATA connections, but I needed three at the same time. So I bought myself a Silicon Image, SIL-3512, PCI Serial ATA Host Controller Card which has two SATA connections, allowing me to connect a further two SATA drives to my PC.
(although the WinMFS process does allow you to copy your TIVO drive as an image, that would require lots of HD space on the machine that is doing the MFS, and so the preferred option, IMHO, is to do the disk-to-disk copying thing, which here required 3 SATA connections on the PC)
The twin-SATA card cost me $35 from a Computer Swap Meet stall.
2. Once I had the PCI SATA card installed, I then had to find two extra power cables for the extra disks, as well as ensure that I had enough SATA cables. On buying the 1 terabyte drive, I asked for an extra SATA cable, so one thing down... but I still needed an appropriate power connection for the Tivo SATA disk and the replacement SATA disk.
What I did here, and your mileage will almost certainly vary, was to remove the SATA-type power from my internal hard disk, and replace it with a conventional 4-lead internal PC power cable, since luckily for me the internal SATA disk could accept the conventional power connector as well as the SATA-type connector.
Also, luckily, this same SATA-type power connector had two plugs on it. This enabled me to power up both the original 160 GB Tivo disk and the replacement 1 TB disk at the same time (as well as still powering up the Windows PC that this was all going to be done on).
After doing those things, and getting myself a decent SATA disk (a Samsung SATA-2 disk with 32 MB cache, 1 terabyte, for $170... from one of the 'Computer Swap Meet" stalls we have around Melbourne), I was able to complete the process. The process really was as simple as registering on the WinMFS site to download the software for free... downloading it and running it – no 'installation' required – and then following the instructions on the WinMFS pages.
The MfsCopy part of WinMFS took about 45 minutes for a 90% full 160 GB drive - yes, it copies the full TIVO drive, including all of your recorded programs. And as noted elsewhere,
WinMFS is single-threaded, so don't bother trying to multi-task while it is running, just be patient and let it run in the foreground.
And yes, again as noted in posts above, you need the Torx-10 and Torx-15 screwdrivers for the Tivo screws... there are two sizes... one for the case's external housing and the platform that holds the disk drive, and the larger size to remove the disk drive from this platform. Got myself a full set of 'Jackly Telecom Tools' including Torx and conventional screwdriver and Allen-key ends, for $25.
So... all up my cost was $230, and about 3 hrs from go to whoa, to go from 160 GB to 1 TB.
The Tivo, once powered back on (the full MfsCopy using WinMFS did retain all of my recordings, and settings for future recordings), did take about 5 minutes to work itself out. So power it on, and leave it for a few minutes before cursing at yourself that it didn't work.

And for those considering this option compared to getting the commercial 1 TB Tivo Expander Drive, my preference would still be on the internal drive upgrade since it doesn't involve yet-another-power-connection at the TV, nor does it mean that my programs are now stored on two disks, married to each other in some funky Tivo way (the internal 160 GB drive and the Expander now work together, meaning that if either one fails, you will lose all of your programs... it also means that the external disk has to remain switched on all the time.) Also, my price for the 1 TB disk, $170, was in March 2009, and so prices will continue to be falling.