IB has divided many communities including our small town in Lake Tahoe,
http://www.truthaboutib.com/usschooldisputes.html 
Here is what happened in Upper St. Claire, PA.
http://www.pittsburghlive.com/x/pittsburghtrib/opinion/columnists/steigerwald/s_434358.html

I have four children who would have been forced into IB so I spent over a year researching the programme.  Most of what I discovered is posted at www.MyInclineVillage.com.

1)  IB will not improve student performance,
Summary - http://scholar.lib.vt.edu/theses/available/etd-04172006-173212/
Entire dissertation - http://scholar.lib.vt.edu/theses/available/etd-04172006-173212/unrestricted/DissertationDeborahRJackson.pdf
http://blog.ednewscolorado.org/wp-content/uploads/2009/12/IB-Plan.pdf
http://www.ibo.org/research/programmevalidation/documents/TXSum.pdf

2)  Over 20 schools drop IB in about a year the most stated reasons for dropping IB are:        
Cost
Lack of student improvement with IB
Less flexible than AP
Lack of participation in IB classes
Lack of college credit for IB

Here is a link to the 20+ schools that dropped IB in about a year,
http://myinclinevillage.com/2011/05/10/how-to-stop-or-remove-ib-from-your-school.aspx

St. Helena CA, just under 1,500 students, is very interesting.  The school board voted unanimously to implement IB.  At least $1,000,000 was spent.  After having IB for less than two years the high school students fought to remove it, stating four of the reasons above.  Three years after the board voted to implement IB they realized the students were right, so they voted unanimously to drop IB.
http://napavalleyregister.com/star/news/local/article_8b306270-4ae5-11e0-b742-001cc4c03286.html
http://napavalleyregister.com/star/news/local/article_93d3c612-5042-11e0-a7c4-001cc4c002e0.html

3)  After all the money spent to implement IB your high school graduates will get less college credit for IB than for AP.  The IB HL classes are two year classes.  Of the colleges that do give credit for IB, most only give credit for HL classes.
http://www.collegeconfidential.com/dean/archives/ap-vs-ib.htm
http://www.princeton.k12.oh.us/HighSchool.cfm?subpage=416

    Reno High vice principal compares AP to IB

http://www.youtube.com/watch?v=TBP3Yx-kDAI

The vice principal states: "Very very clearly AP offers more college credit than IB.  Universities by in large will give far more credit for passed AP courses than they will for IB courses.  We did a little research and we can tell you that over 200 universities actively prefer the AP program over IB vs. only four universities that prefer IB over AP."  Then the 200 universities that prefer AP are displayed. 

The IBO website use to say IB was “college prep.”  It was recently changed to, “The (IB) Diploma Programme fosters the knowledge, skills and attitudes that enable students to excel in university.”  http://www.ibo.org/recognition/
AP classes are college level classes,
http://www.collegeboard.com/student/testing/ap/about.html

When schools attempt to combine IB and AP in one class, per the IBO, the IB curriculum must take priority.

What high schools (especially small schools) discover - after spending a lot of money to implement IB they inadvertently increase college costs for most of their graduates.
 
4)  The fact that IB is a UN sponsored programme is an issue for some parents,
http://myinclinevillage.com/2010/07/16/even-jay-matthews-admits-colleges-give-more-credit-for-ap.aspx
 
5)  If students are being forced into IB and parents have religious objections (and possibly political objections) to the  programme, your District could be at risk for a law suit.
http://www.aclj.org/Issues/Resources/Document.aspx?ID=15

6) Once implemented, IB can be very difficult to remove.  Here is one of the law suits filed because of IB.  In this suit the school board voted to remove IB, the ACLU got involved and forced the district to continue to offer IB. 
http://www.pittsburghlive.com/x/pittsburghtrib/news/cityregion/s_447148.html

There are many other issues with IB some of them can be found at,
www.MyInclineVillage.com