And Abram passed through the land unto the place of Sichem, unto the plain of Moreh. And the Canaanite was then in the land.
As many have noted, this verse appears to have been written* by someone living after the Canaanites had ceased to be in the land, that is, long after Joshua's conquest and the subsequent settlement. No less an authority than the Ibn Ezra, in fact, suggests that these words might not have been part of the Torah Joshua took across the Jordan.
Here's another possibility, wholly my own:
In Hebrew the word for "was then" is oz, with an aleph. If you spell the word with an ayin, however, its meaning changes. Instead of translating to "...and the Canaanite was then in the land." the phrase v'hacananee oz b'aretz becomes "...and the Canaanite were strong in the land."
As it happens, this emendation fits beautifully with the very next verse, Genesis 12:7 where, for the very first time, God promises Abraham that he and his children will posses the land.
Why was the Divine Promise stated here, and not earlier? Because Abraham had seen that the Cannites were strong and in the land, and he worried that they might one day oppress or assimilate his descendants. Therefore, God offered him reassurance in the form of this promise.
* Correction, November 15, 2005: Due to an editing error, the sentance marked with an astrick originally said "As many have noted, this verse can only have been written by someone living after the Canaanites had ceased to be in the land." As the first commenter pointed out, no one (no one Orthodox anyway) says that this is the "only" interpretation.
Apologies to Gil who I now see blogged inadequately on this subject last Sunday. I should have given him a [related] at the outset.