At worst, it looks like the Democrats will retain their majority in the Senate, but with a much lower margin. At best, the Republicans could gain an outright majority. This is more likely than the poll results indicate, as some of the constituent polls are clearly biased toward the Democrats, probably because of likely voter sampling error. The California polls may be biased toward the Republicans, because young voters may vote in higher-than-expected numbers to support the marijuana legalization initiative.
There's another interesting possibility that emerges if the Democrats end up with a one or two seat majority (and this is actually looking like quite a likely outcome). In that environment, Senators who currently caucus with the Democrats have a huge incentive to switch to the Republicans, as they can likely cut an interesting deal to incent them to switch. For example, one could imagine Lieberman getting chairmanship of a committee relating to foreign affairs or military interests in exchange for switching.
Next Tuesday night will be a cliffhanger. I'm not planning on getting much sleep!