Last year, we wrote about how Google is to perceive subdomains. Matt Cutts wrote back then that in some circumstances, subdomains will be treated like subfolders. He goes into more detail on his blog. In the post, he explains that sometimes people have performed long tail searches and gotten too many results from a single domain and its subdomains and Google has made it more difficult for that to occur.
The issue was relatively silent for a year, but now people are starting to ask questions. On a WebmasterWorld thread, tedster says that "A legitimate subdomain today often gets treated as something of a hybrid between a totally different domain and a part of the main domain."
On the other hand, there's a DigitalPoint Forums thread that disputes the weak relationship:
I've actually learned first hand that Google started treating subdomains as directories (happened months ago). I had some websites on subdomains where the main domain had 0 links. The websites were doing well until one day when they fell drastically. I then started to build links to the main domain and within 2 months the subdomains were also stronger than ever. The stronger the domain is - the stronger the subdomain is.
That said, there's confusion yet again.