A search of the web reveals that many institutions across the country have struggled with the decision to adopt a +/- grading system. In the process, they conducted research and reviewed the research on the perceptions and actual impact of +/- systems.
That literature also discusses the potential negative impact of the C- grade (dropping grade points below 2.0 that impact financial aid, scholarships, and program probation). Several systems have reasoned that since there is no A+ (4.3) that the C- will not exist - so there won't be more - grades than + grades. This eliminates the problem of the C- student falling below the 2.0 gpa requirements. This, by the way, was the conclusion and current policy for the California Community College System:
http://www.academicsenate.cc.ca.us/publications/Papers/Plus_minus_grading.html
Another report focusing on the problems that +/- created at Missouri University - Columbia can be found at:
http://www.missouri.edu/~bkuc97/plusminus.html
Seton Hall recently (2003) adopted the +/- system without an A+ or a D- grade.
An excellent review of the literature is found in the UMBC study:
http://www.umbc.edu/ugc/Plus_Minus_Summary.htm
A study in North Carolina (listen up y'all - Jerry S.) concludes:
* Most peer instituions have shifted to or are considering +/- systems
Trend is in direction of plus/minus systems
1,578 institutions: 46% whole letters; 36% +/- ; 6% + only
* Students initially oppose the changes - believing that GPAs will drop
* GPAs are actually unchanged - with the exception of the top 10% of students who experience about a .1 drop because of the impact of A- grade (most institutions don't provide A+ bonuses)
* Students adjust to and accept the new systems
Some campuses conduct a year or two of "trial" before actual use of the +/- grades in the calculation of the GPAs. This permits students/faculty to measure the potential impact of the new system before it actually affects anyone's GPA.
The general advice seems to be: take your time implementing the shift.