Great entry: Starting a Novel in the Wrong Place by agent Kristin Nelson (blog link to the left).
I'm SO guilty of this -- starting a novel where I "think" it should start, when actually, it should really start about a chapter later. Those introductory pages are crucial. They serve multiple purposes - doling out pertinent information, setting an instant tone/POV/setting, and also hooking your readers - getting them interested from the very first sentence. Not an easy task. In fact, I do think I tend to spend more time on the first 5 pages of my novels than any other set of 5 pages throughout.
En medias ras, I always tell my students - start your novel, your story, "in the middle of things." Whether it's the middle of a conversation, or an activity - put the reader smack dab into the middle of something, and then fill in the gaps later - those important things like setting, context, background info - these details can all be weaved in a tiny bit later. And, doing so can create a bit of mystery for your readers. Let them work for it...
Anyway, Ms. Nelson says it so much better than I could in that entry, and I agree with her 100%. Excellent advice!