I think that is normal when SERP is down..just update and link building I'm sure your SERP will be back
No need to worry at all. During a PR export, Google needs huge amount of resources even by its standard and hence there is wild fluctuation in SERPs. Just let everything cool down, keep adding valuable contents and wait. Your SERP will be back in a vengeance. Good luck.
Most probably due to some algo changes, other sites have now gain significant ranking position and that push your site down. Although I would like to believe both PR and serp are not related altogether.
FYI All, update on this situation. This morning, the same site which lost so many positions the other day corrected, and now is on page 2. 16 position gain and within sight of page 1 for primary keyword! I guess that proves it was just an aberration with Google algorithms during their PR updates. Happy new year all!