I've been actively building links to my site for a while and the activity has increased quite a bit since this month. This includes directory submissions, article marketing, wiki links and so on. I noticed that my site's PR dropped from 2 to N/A and i'm wondering if this is an 'initial warning' of sorts from Google? The search seems to be fine from Google and I rank in top 10 for some keywords as well. Kindly advise.