I have a website set up with Google Analytics. I also have a blog that is a sub domain (actually my website uses "www.mydomain.com" and my blog uses "blog.mydomain.com").
I am wondering if I should use the same Google Analytics tracker ID for both my website and the blog, or if they should be separate. Is there any benefits either way?
GA provides very flexible analytic tools for you to filter data specific to certain sub-domains, so there shouldn't be any particular reason why you would want to split it into 2 profiles/GA ID.
However if you intend to 'share' the GA access of one sub-domain to someone else you probably want to create another profile for it.
Having said that, there's no harm splitting the two if you want to view the reports independently. I'm not sure if GA currently provides merging data of 2 or more profiles, if not, a consolidated report of all main/sub-domains might be a hassle.
Ultimately, it depends if you want to consider your blog separate from the rest of your site. If you want to track the analytics separately, make a new profile for it. Otherwise, install the tracking code for a subdomained site into both domains to track them together. You can get the embed code from Google Analytics the same place as you'd find the regular code, just change the option to a site with subdomains.