The several uses to which the <title>
tag is put complicates the question…
As browser window or search result titles, it would seem to make little difference — there's enough room for everything and the display order is unrelated to the title.
For tab and taskbar / dock titles, space is at more of a premium. Placing the page name first makes it easier to distinguish between multiple pages on the same site, but if the page names are generic ("Search Results", "Intro to CSS"), it may be more important to distinguish between sites rather than between pages.
In bookmark titles, having the site name first will both make them easier to find and allow you to group them by doing a simple sort, but users can easily alter the title to their liking (and often do), making the provided "default" title less critical.
What other factors should be considered? Is there a commonly-accepted "best practice"?