Splog's have a distinct pattern... namely large amounts of links from comments. Akismet, an optional wordpress plugin that centrally tracks and filters spam on comments to wordpress blogs easily detects splogs on the wordpress system by the large amount of comment links to them and can then kill the splog.
That said... I use google's blogger.com as my primary blog, and while I see evidence there are a tremendous amount of splogs on blogger I've had FAR more success with blogger's spam filters then wordpress. Even though wordpress has filtered out splogs comment spam on wordpress seems particularly virulent.
I guess what I'm saying is different services excel in different areas.
From: PlagiarismToday - Why Wordpress.com is Virtually Spam Free
A recent study by WebmasterWorld found that an estimated 77% of all blogs on Google’s Blogspot service were spam. Similarly, AOL Hometown, had well over 80% of its results turn out to be spam. Even MSN Spaces, which as not mentioned in the report, is claimed to host an estimated ten percent of spammer Web site.
It seems as if nearly every major free blog hosting service has been either overrun or nearly overrun with spam. However, one services stands alone, a relative oasis of spam cleanliness, Automattic’s Wordpress.com.