A recent wave of polling and Senate announcements actually provide us with a decent public record of current public opinion in key Senate races in 2008. The following are all currently Republican-held seats, and they are ranked in order of how good they look for Democrats
I have been unable to find any polls for Alaska, Nebraska, and Tennessee, which also might end up being competitive. I also have been unable to find any polls featuring the two best Republican pickup opportunities, South Dakota and Louisiana.
I have to say, I have not seen such a wide range of potential targets for one party since 1994. Democrats currently have fourteen realistic pickup chances right now, provided that we get good candidates in Alaska, Nebraska and New Mexico (for the latter, I am projecting a Udall run right now). Some of the longer shots are balanced out by excellent Democratic chances in Virginia, New Mexico, New Hampshire and, to a somewhat lesser extent, Colorado and Minnesota. Beyond the top two tiers, Kentucky and Oregon also look like strongly competitive races, while North Carolina and Texas are real up and coming possibilities. Not a single Republican on this list should feel comfortable. About the only thing that really surprises me about this list is that Maine doesn't even make the top ten in terms of current poll competitiveness.
A massive Democratic wave in 2008 could spell the end of "Democrats need to do X in order to win" literature for the foreseeable future. The electoral difficulties that Democrats have had to live with for a few decades now would be over, and the first, solid, post-civil war "natural" Democratic majority without the remnants of the "solid south" would be achieved. The would be quite an achievement, and a real sea-change in American politics. Even if it does leave the Pretenders in charge, it should still be sufficient to break the working conservative majority on a decent range of policy areas.