Redstate has a very comprehensive review of the 2006 senate elections and potential republican pickups. Like us here they see a 3+ seat pickup easily within the realm of possibilities.

Two days after the 2004 Republican win, I stated that a filibuster-proof Senate was possible in 2006. Although unlikely, the possibility of Republicans picking up 5 seats is not out of the question.
I further asserted that we might see an exodus of incumbent Democrats since there is only a dim chance of re-acquiring majority status in the short to medium run. This hunch so far seems to be coming true as Senators Sarbanes (MD) and Jeffords (VT) have declared retirement, Senator Corzine (NJ) sees more value in running for Governor than staying in the Senate, and Senator Dayton (MN) decided another close race was not worth his time. The only Republican to signal his departure so far has been Senator Frist who is honoring a term limit pledge. Minority Leader Reid’s comment that it would take a “miracle” to win back the Senate in 2006 reinforces the fact that Senators in the minority are probably stuck there for the near future merely voting “No” on legislation instead of pursuing their own agenda.