Here is a good piece in the NYT this morning looking at the strategies McCain and Obama are already developing. Unsurprisingly, independents and Hispanics will be important to both camps. Obama has seen some support among independents slip since the Wright controversy and has trailed Clinton in the Hispanic vote in most primaries.
The Obama camp will portray McCain as a conservative that isn't the maverick or independent-minded politician that is usually associated with him. In particular, they will harp on McCain's opposition to abortion and gay rights.
The article highlights southwest states - Colorado, New Mexico, Nevada, maybe Arizona - as battlegrounds this year and also mentions that McCain will likely target Pennsylvania and New Hampshire, two states where Obama has lost primaries.
I don't think the importance of the Hispanic vote in this election can be over-emphasized. While it remains to be seen just how much of the rapidly growing Hispanic population will turn out (or how the recent voter ID SCOTUS decision will affect their ability to vote - but that is for another time...), this block will develop into a critical part of the Democratic coalition. Unfortunately for Dems, McCain is the best candidate the GOP could dream of to court them this year because of his more open-minded stance on immigration and his work on the legislation that the GOP killed last year.