Um. It is a piece of electronic equipment. It would be not that must of stretch to assume it would be updated for the Christmas buying period. How many products are updated every year before Christmas?
Looking at the history of Canon's DSLR lineup: none. The 400D was released late August 2006. The 300D was three years prior to that, late August 2003. The 5D, late August 2005. 20D, late August 2004. 1D, November 2001 (and let's face it, how many people are going to buy their loved ones a Canon EOS 1 series camera for Christmas?)
If you think the 400D is a good fit for your needs, by all means grab it. Even if Canon announces its successor tomorrow (which I would bet very heavily against), it will still take excellent pictures.
Seems that a lot of people forget that the release of a new product does not automatically render the old product useless. My 20D is still taking good pictures (although it needs a sensor clean ... sigh), and I feel no need to upgrade to the 30D (truth be told, I'm more likely to upgrade to the 5D's successor, but there's a lens purchase to make before I do that, meaning it's at least a year away, probably two to three to five.)
Summary: if somebody offered me even money that Canon would not release the 400D's successor this year, I'd take it. Unless they were in Canon's DSLR R&D department.