This behaviour is a bug in the 2.5 betas. The text boxes should take the margins set in the document settings dialog into account. There are several ways to work around it.
1: Use a 1-up imposition. This will create a layout with the margins set in the document settings dialog, but it may scale the contents down slightly.
2: use the inspector to size the text box down a bit, and then center it on the page. This *could* cause some of the text to fall off the end of the box, if there is more text in the box than can fit the new, smaller, size.
I’ll work on a fix for the final release of version 2.5.