This is the final year... In order to comeback they must outsell the 360 on a worldwide basis by January 1, 2010. If this is not happening then a comeback becomes extremely unlikely.
As for comeback on the Wii that is dependent on one thing... Does the Wii continue to stay popular amongst casuals? If it does, then a comeback on the Wii is highly unlikely...
The problem I see developing is that there appears to be a "secondary generation" developing... Wii HD, 360 with Natal, PS3-2 that addresses the under the cover issues that developers have had with the PS3 and adds their motion controller. All systems will be backward and forward compatible thereby extending the generation rather then truely ending it. In other words you can stay with what you have and it will work, but the secondary generation will give you what you really want.