I'll take a quick stab at last generation's numbers. Let's say that 1% of all consoles each year are replacement units of all units sold to date. (Could be due to a hardware failure that was due to poor design, excessive use, accident, etc. This also assumes that some people may choose to not replace their systems or buy a used console as a replacement.) If we look at sales each year, we'd get a picture like this:
March 2000 - PS2 - 1.41M - 14K units sold as replacements
March 2001 - PS2 - 10.6M - 106K
March 2002 - PS2 - 28.7M - 287K - GC - 3.8M - 38K - Xbox - 3.1M - 31K
March 2003 - PS2 - 51.2M - 512K - GC - 9.6M - 96K - Xbox - 8.6M - 86K
March 2004 - PS2 - 71.3M - 713K - GC - 14.6M - 146K - Xbox - 14.5M - 145K
March 2005 - PS2 - 87.5M - 875K - GC - 18.5M - 185K - Xbox - 20.7M - 207K
March 2006 - PS2 - 103.7M - 1M - GC - 20.9M - 209K - Xbox - 24.2M - 242K
So through March 2006, that would provide a conservative estimate of 3.507M of the PS2's 103.7M shipped number as replacements. You'd have 674,000 GameCube's replaced and 711,000 replacement Xboxes. This is of course a real rough estimate and there could be a number of other factors that play into these numbers including the redesign of the PS2 (which probably swayed a few people to buy a new console even if their system was already working), actual failure rates of the systems, price of a used versus new system, etc.
A more aggressive estimate may assume that a greater percentage of systems fail as they get older. Assuming 1% of consoles fail within the first year and that number increases 1% each year (so 2% of total system sold two years ago fail, etc.), you'd have numbers like this:
March 2000 - PS2 - 1.41M - 14K units sold as replacements
March 2001 - PS2 - 10.6M - 120K
March 2002 - PS2 - 28.7M - 421K - GC - 3.8M - 38K - Xbox - 3.1M - 31K
March 2003 - PS2 - 51.2M - 1.05M - GC - 9.6M - 134K - Xbox - 8.6M - 117K
March 2004 - PS2 - 71.3M - 2.17M - GC - 14.6M - 356K - Xbox - 14.5M - 324K
March 2005 - PS2 - 87.5M - 3.09M - GC - 18.5M - 637K - Xbox - 20.7M - 617K
March 2006 - PS2 - 103.7M - 6.64M - GC - 20.9M - 1.13M - Xbox - 24.2M -1.12M
This formula indicates that 13.5M or about 13% of the PS2's 103.7M shipped number were replacements. (2.3M for GameCube and 2.2M for Xbox.) Of course this assumes that 28% of the consoles Sony sold before March 2000 have since been replaced.
I believe the actual numbers lie somewhere in the middle of both estimates. While significant, I still don't think that really puts Sony's dominance of last generation in question. With this generation's console war still young and close, every unit counts.