I found no reference for a bounding box for Group object, but when using the latest Dojo version, getBoundingbox returns null for Groups as well
Is this problem related to this bug fix? Sounds like it to me. It was Changed 7 months ago by pruzand and marked as fixed
http://bugs.dojotoolkit.org/ticket/7019
Update 2012-07-24: I downloaded release-1.8.0rc1 locally and setup the same test in the resolved ticket. This indeed has been fixed in 1.8 and getBoundingBox returning null on groups is resolved as stated in the ticket. The test script successfully returns:
bbox of group: {"x":0,"y":2,"endX":238,"endY":379,"width":238,"height":377}
transformed bbox: [{"x":277.7302117816355,"y":152.5448441073243},{"x":512.114456998541,"y":193.87311039205372},{"x":446.64909401810826,"y":565.1456332776561},{"x":212.26484880120276,"y":523.8173669929267}]
Also for what it’s worth, according to this twitter post, 1.8 is due to be released in a week or two.