I think the PS2 dying could negatively affect the Wii. The PS2 doing well means low end games development is more viable.
I think the PS2 being discontinued would mainly positively affect PS3 sales, but would be bad for Sony's gaming devision in terms of profits. I think they should continue at keast until a 299 dollar/Euro Slimline PS3 is ready for the mass market. The Wii install base can continue to grow and stand on its own feet, 1st party games sell well but a larger user community is needed for 3rd party developers.