I am in favor of pushing D7 later, not for testing (which I am not very keen on, due to past experience with automated testing), but for the reasons Larry outlined here:
The slow uptake of D6 is due mostly to everyone and their brother taking the opportunity to rewrite their contrib modules at the same time as upgrading to D6, or right before doing so. Views, Panels, Project*, CCK, filefield, imagefield, and a half-dozen others have major improvements planned in contrib space that are progressing; they just take time. Those, in turn, block dozens of other modules.