Right now, Google is the default search engine on the iPhone. You can go into the Preferences on the device, and switch the engine, but you only have one choice: Yahoo. Most, of course, do not make that change.
Naturally a move away from Google and to a different search engine might be something Apple would love to do. After all, at the recent Google I/O conference, Google execs wasted little time bashing Apple. At the same time, Steve Jobs has said "Make no mistake: Google wants to kill the iPhone. We won’t let them.”
Google has other services on the iPhone, such as mapping, however. Given the strain between the two companies, it's in Apple's best interest to consider other options. Will Apple dump Google for search? That's doubtful, as most consumers think Google for search. Mapping, on the other hand, might be more likely.