Wine is very compatible if it's used in conjunction with a real copy of Windows (Wine has the capability of using the real DLLs instead of its own cloned versions.) The only serious issues are with those applications that need to address hardware (such as USB drivers), including those that rely upon copy prevention systems.
Apple just bundling Wine wouldn't work, but if they created a Windows installer that also configured a version of Wine to work under Mac OS X, then they could produce something that "just works" in the majority of cases. They'd have to do some work to overcome the problems caused by copy prevention systems, but otherwise...
I've been using Wine since I started using Linux in about '94. It's nothing worth writing home about at this point in time. The OS formerly known as Lindows quickly abandoned their hopes/dreams of their Linux OS running "virtually anything" Windows, without Windows, a long time ago, for a very real-world reason (they couldn't handle the support for every Windows app ever written, and their claims of compatibility bordered on false advertising.)
Apple would be nuts to bundle a version of Wine and market it as allowing OS X 10.5 users as being "able run Windows, without paying for Windows" for a variety of reasons.
Real World (for Joe Average Computer User):
If you absolutely need 100% functionality of a given Windows program, your best bet is to simply boot Windows on a machine that provides full hardware supports for the OS.
If you can "get by" by simply running "many" (but not all) Windows apps, you buy Parallels.
If you can "get by" with even fewer, e.g.,
some apps, like IE, most of MS Office, and have a great deal of time to muck around with getting apps requring DX10 ('gamez!') to run, then you could go the cheap-thrills route, and use Wine.
This is, as you already knows, not to be taken as a sign that I think Apple will incorporate such a technology into Mac OS X. I honestly don't think anyone's barking up the right tree with the Bootcamp/Windows speculation. I think the Apple press release can be taken at face value, this is iPhone related.
Apple resources are being stretched thin, with good reason (iPhone, OS 10.5, new hardware in '07, etc.), I don't know why it's so difficult for people to accept that reality.