Don't expect to see a Penryn before MWSF 2008. It is doubtful that they will be readily available too much before the end of the year and Apple will not make any lineup changes past early November in preparation for the holiday season.
Seeing as Penryn is also slated for the Santa Rosa chipset, I would expect to see Santa Rosa iMacs and notebooks this May - July and obviously 8-Core Mac Pros anywhere between NAB and mid-May.
Then that would allow for the next round of iMac, notebook, and Mac Pro updates to begin next Jan and go through March or April of 2008.
Personally I plan on getting a Leopard Mac as soon as available, and potentially upgrading to Penryn 9-12 months later. The real question is will we see any touch-screen technology this year, most likely not, Penryn's 45nm chips, speeds and power are a more likely candidate, if that soon. We may only see an iMac color option this year with any more substantial redesign coming with the next generation CPU. And even then it is unlikely to be as dramatic as the lampshade to chin progression.