Confirmed on OS X 10.6.8 MacBook Air 11-inch
Other users reported the same results but I do not know their system specs.
Broken: 2.70a f93bc76
Worked: May have worked in 2.62.1 - based on the training DVD "[[ http://cgcookie.com/blender/cgc-courses/interior-3d-architectural-visualization/ | Interior Architectural Visualization ]]" Chapter 4: "Modeling the Trim"
In this tutorial the author is able to Toggle Cyclic while the curve is 3D without any pinching at the corners.
The results that myself and other users are now getting are different from what is demonstrated in the training DVD.
**Short description of error**
When using a Bevel Object along a path Curve, pinching occurs at corners.
I consulted with some other users and [[ http://blenderartists.org/forum/showthread.php?336482-Uneven-thickness-when-using-a-Bevel-Object-along-a-path&p=2644719 | it was suggested this could be a bug ]].
Oddly, switching the path Curve from 3D to 2D fixes the problem, but in doing so one obviously becomes limited in what kinds of geometry can be made.
Note that if the path Curve is Cyclic (closed loop) the pinching occurs evenly at each angle of the path and is less obvious, but one can still see a difference in thickness if setting the curve from 3D to 2D.
**Exact steps for others to reproduce the error**
1. Load the sample .blend
Add Curve > Circle, then Add Curve > Bezier, set its handles to Vector, in curve properties set the BezierCircle as the Bevel Object.
2. Change the path Curve from 2D to 3D and the problem goes away. Thickness becomes uniform, but 2D is restrictive.