Latest bellwether states which aren't definitively a base state for either party:
Ohio—Unbroken streak since 1964; statewide margins within five points from national beginning with that election and including up to the most recent in 2012. It's voted for the winner in every election, except 1944 and 1960, since 1896.
Florida—Statewide margins within five points from national beginning in 1996. State has voted for the winner in all elections, minus the Democratic pickup winners of 1960 and 1992, since 1928.
Virginia—Its margins in 2008 and 2012, both in President Obama's column, came closest to the national numbers.
Colorado—Its margin in 2008 ranked it No. 2 in mirroring the national margin for Obama; in 2012, it came down to No. 3, with Ohio at No. 2, but since 1996 the spread between Colo. and Va. have been no greater than 3.53 percent (from 2004); they're commonly a point or two from each other in their margins.
I do want to note this: Go back to 1912, when bellwether New Mexico and Arizona (which voted for all winners in each cycle of its first five decades), and you have that year the established 48 contiguous United States. From 1912 to 100 years later in 2012, there have been 11 elections where the winner carried at least four of ever five states. (Prior to Alaska and Hawaii, in 1960, carriage of 39 states accounted for 81 percent.) During this period the following won landslides with carriage of at least 80 percent of the available states: Woodrow Wilson, 1912; Herbert Hoover, 1928; Franklin Roosevelt, 1932 and 1936; Dwight Eisenhower, 1952 and 1956; Lyndon Johnson, 1964; Richard Nixon, 1972; Ronald Reagan, 1980 and 1984; and George Bush, 1988.
Since 1992, no winner has carried more than the 32 states won with the first election that year of Bill Clinton. In so many of those other election cycles, there appeared to be a lot of states in the bellwether category. Nowadays, the two parties aren't trying to win landslides on this scale. So, the talk of bellwethers, with make them sound especially special, has become the norm.
Basically this minus Florida, IMHO.
The "Tipping Point", which is basically the perfect bellwether point currently lies at about 1.6+D, the same as Colorado, which was the tipping point state in the 2012 and 2008 elections.
I think that Florida is a tad too far into the Republican column to be considered a bellwether state anymore. The Democrats could lose Florida and still win the election with relative ease as long as they take Colorado or Ohio, both of which are more Democratic than Florida.
Ohio is definitely a bellwether state.
Colorado was the tipping point for the last two elections, so currently it's the most obvious bellwether. It might lose it's status in the coming elections, though, if it continues to trend Democratic.
As Virginia moves closer and closer toward the tipping point it will become more and more of an important bellwether.
The only up-and-coming bellwether I can think of is maybe Pennsylvania if it continues to slide from the Democrats. Wisconsin or Michigan are dubious, I think there's a good chance their 2012 Republican trends were flukes, considering in 2004 and 2008 both were trending Democratic.
