It looks like Windows Vista SP2 will be squeezed out not too long before Windows 7 itself hits. TechARP, who originally reported to expect it in April, says its release date has slipped to May.
The first SP2 beta actually dropped last month. Here's a complete list of everything that's in it, including native Blu-ray recording and better Wi-Fi configuration. Like the last Vista service pack, it's a bunch of fixes and minor tweaks, not a feature bomb.
Vista isn't particularly aching for a service pack, so the only material effect of the delay, really, is that it further highlights Vista's impending irrelevancy—the looming moment people are waiting for to cast it off, both literally and figuratively, if unfairly—because it pushes SP2's release date to within two months of the expected final release of Windows 7.