First of all there are two PageRank indexes. The first one shows a number in Google Toolbar, characterizing the PageRank of your page and is updated every 3-4 months. Another PageRank is always updated and is used in search engine ranking algorithm.
The first one is used only by web-masters for rating indication and the second one directly affects the search return.
But let’s get back to our question: Does Google PageRank depend only on the amount of inbound links?
The answer is: No. Of course the number of inbound links is important but Google cares about their quality and relevance.
The quality means trust of the sites linking to you. For instance a link from Stanford University will be of greater quality than links from your sister’s blog.
Relevance means the content closeness of the linking site to your content. If your site is about cats then the link from your sister’s blog who writes about cats will be more relevant than a link from Stanford University quantum physics page.
Take a look at another example. Let’s think you are running two blogs: “Blog A” and “Blog B”. Both of them are about IT. You buy about 30 links from non-relevant low PageRank sites for your “Blog A”. And for the “Blog B” you write one good link that has naturally got the inbound links.
If you wait for three months and will not change your blogs there is a great probability that the first blog will not get PageRank and the second will have PageRank 1 or higher!