So I really enjoyed Dead Rising 1, and plan on getting Dead Rising 2 (weapons combinations = stupid fun). Anyway, how necessary is it to the continuity of the story to play through Dead Rising 2 Case 0? In that, how much information does it reveal that you are not likely to glean throughout playing the actual game...?
From what I've gathered, Case Zero is a great "appetizer" for Dead Rising 2. I don't believe it has anything that is crucial to DR2's storyline. I don't think Capcom would have kept it an Xbox 360-exclusive extra if it did. Although, the next game, the epilogue case, Case West, which will also be 360-exclusive, will have crucial plot points for a DR1 player (the return of Frank West). (But that exclusivity makes sense given DR1's own long 360 exclusivity.)
That said, I've heard that Case Zero is a great introduction to DR2, particularly for new and on-the-fence players, and to DR2's new mechanics. Experience you gain in Case Zero gets applied to DR2, and I've heard that Case Zero alone provides a lot of interesting content with good re-playability.
As a DR1 fan, the answer most likely is that you don't "need" to play Case Zero, but that you probably want to, just because it will be fun. It's also a good first purchase just to double check that all of the changes from DR1 to DR2 are as fun and exciting as you expect.
Story-wise, this is a prologue. A spot of car trouble on the way to Fortune City. Gameplay-wise, this shows you a sampling of the Dead Rising experience, complete with crafting, improved survivor AI, and a pretty good demo of the whole zombie-stomping experience.