Knowing the way Sony is and has been dragging things out, PS3 has a good chance of lasting at least 10 years.
Look at Playstation. There was the original gray box, then came the PSOne with had a few upgrades to keep it selling a bit longer. I don't know what the specific upgrades were (besides like an add-on screen), but they helped.
The Playstation 2 transitioned to the PSTwo Slim, which has kept it going on pretty long, lasting up to even now (8 years). When will it finally die? It's good to know that it can even last this long.
The Playstation Portable was phat, then lost some weight, making the PSP-2000. This SlimLite transformation has, and will, keep it living for a while. Especially since it's part of the Portable Industry.
The PS3 is part of the HD generation, the time when people are becoming accustomed and comfortable with near-perfect graphics and media capabilities. Not to mention that Blu-Ray is helping the HD cause. People can feel "settled" now that things are becoming so powerful (processing/technology wise), and we can't find much more room for improvement... for a while. And as if this wasn't enough to help the PS3, like its previous brethren, Sony will release an upgraded PS3 (slimness, slightly more power, etc.) that will make people extremely comfortable and satisfied with buying it in the HD era.
I won't even get started about the 8th console generation, because I'd get too confused. And I also believe that the PS3 can live at least 10 years because it's already past this generation, keeping it alive until other, newer consoles catch up (by the time the "Wii2" or "X-Box720" come out, they'll probably be as strong as the PSThreePointFive Slim Thingy). Remember? Us PS3 owners don't have shit for games (yet) because most, if not all the developers say things such as "the PS3 is too hard to develop for, as the Cell is too complex and advanced." Obviously this means the full capacity of the PS3 hasn't been tapped yet, and with its very limited power so far it's going to last well more than 2 years. So how long could it last when all of its processing power is utilized and developers can make super-games? Time will tell.
Sony's gonna keep kicking ass. Especially with the PS3-HDMI-BRAVIA combo. *Orgasm*