Quick Vermont facts

Congrats to Vermont for legalizing same-sex marriage!
What I could figure out with a quick check:
Text of the bill.
It goes into effect on September 1, 2009. A while to wait for Vermonters, but early Vermont September should produce some beautiful wedding photos.

Longevity prospects: Excellent. The Vermont constitution is really hard to amend, and cannot be amended by citizen initiative:

The Vermont General Assembly has the sole power to propose amendments to the Constitution of Vermont. An amendment must originate in the Senate, where it must receive a two-thirds vote. After passing the Senate, it must also receive a majority vote in the House. Any amendment that passes both Houses, must be repassed by majority votes, after a newly elected legislature is seated; again, first in the Senate, then in the House. The proposed amendment must then be passed by a majority of the state's voters at a referendum. Only every other Senate session may initiate the amendment process. Thus, Senates elected in off-year (i.e. non-Presidential) elections may initiate amendments, but not Senates elected during Presidential elections. (Vermont Constitution, Chapter 2, Section 72)

(From Wikipedia, but http://www.vermont-archives.org/govhistory/governance/Constitution/amending.html agrees.)
So the anti-marriage groups would need a significant legislative turnaround in both houses to repeal this.