Next-gen will start around 2011. This is unless the Wii takes a nosedive in 2008-2009 due to weaker hardware causing some sort of unintendend lower sales (as I believe it will).
Should that be the case, I think Nintendo could first strike in late 2010, or early 2011.
If that doesn't happen, MS will most likely have the next Xbox out (provided they made a profit on the X360), in 2011 or early 2012.
Next gen should start around 2011 - 5 years from the last current-gen system made, and 6 years from the earliest. That doesn't mean every manufacturer will have a new system out by then. If anything, it could last an extra year due to the great specifications of the X360 and PS3 taking longer than normal to get all their juices out.
Back from the dead, I'm afraid.
