I've been meaning to ask this for some time now.

I look at my log files and check to see what pages the 3 main spiders look at. I understand when they do a full scan or when you can see them drilling down from the root or other page.

Most often it is pages here and pages there sort of at random.

Does anyone have an idea of how they decide what pages they want to see?

I know that Google does not know what you click after a search. Did a packet sniff.
Maybe all the URLs of the first page?
Maybe from the Googlebar (if installed) after a search?

Assuming your layout is consistent throughout the site, a very general rule of thumb is "the greater the number of clicks away from the home page, the longer it will take to crawl and index."

For the most part, not much goes on with a new site in the first month. It's usually the home page and maybe 1 or 2 other ones (quite often, it's the first couple of links but that's not always the case). Wait about 3-4 months and see if your site is fully indexed, and then worry if it's still random.

Despite their name SE "crawlers" do not crawl. They simply are sent out from the SE datacentre to go and get the source code delivered from a URI.

Well it depends upon which page is easily accesable, I mean the pages having the more quality links are indexed more than others.

Akash