Are referendums that would divide up the electors proportionally
according to the state vote in a presidential election in any way
unconstitutional?  If a proportional such a referendum passes and it
was appealed to the Supreme Court to determine if it is
constitutionally allowed, how would the Supreme Court rule?

Actually, the Constitution already provides for state legislatures
allocating electors according to any procedure they deem appropriate. 
Therefore, the Colorado proposal to allocate electors proportionally
to the state's popular vote would presumably be constitutional. 
Already, Maine and Nebraska depart from the conventional "winner take
all" format used in the other 48 states, with both having done so for
several years.

In the absence of any obvious issues of constitutionality, it is
doubtful the Supreme Court would have any reason to object to such a
procedure and would likely not agree to hear such a case.

"The U.S. Constitution- in Article II, Section 1, clause 2 (which was
not altered by the later 12th Amendment)- reads, in part, as follows:
"Each State shall appoint, in such manner as the Legislature thereof
may direct, a Number of Electors... etc." [italics ours]; it is this
constitutional provision which permits the several States to do what
Maine and Nebraska have already done in switching over to the
"districting" system from the more usual so-called "general ticket"
system for allocating electors."

"The candidate with the highest popular vote tally receives all of the
state's electoral votes, with the exception of electoral votes from
Maine and Nebraska."

"In Maine and Nebraska only 2 at-large electoral votes go to the
winner of the statewide popular vote: in these states, the
presidential candidate with the highest popular vote in each of the
state's Congressional Districts wins 1 electoral vote from that
particular district as well. Maine has been doing this since the 1972
presidential election, Nebraska is a newcomer to this "districting"
system of allocating electoral votes to the presidential candidates in
the November General Election- having had this in place only beginning
with the 1996 election."

Sincerely,

Wonko

Source: "Electoral College 2000" The Green Papers (February 7, 2001)
http://www.thegreenpapers.com/Electors.html