my guess is that it won't happen until the end of the year

I'd imagine the 10D successor will have some specs comparable to the 1D Mark II, like an 8 mpixel sensor, but probably not support the 8 frames/sec and 40 frames in the buffer of the 1D Mark II. Heck, I'd be thrilled with 3-4 frames/sec and a 10 frame buffer. If (big if) they're using the same sensor on both camera bodies, then it would make sense for the initial production run to go to the 1D Mark II and for the 10D successor to come out in time for Christmas.

On the other hand, you could argue that Canon was smart to get the Digital Rebel (a.k.a. EOS 300D) to market months before Nikon has a D70 to compete with it. I'll bet they want to do something similar in the next market segment, as Nikon will, at some point, have a successor to the D100.