There is an excellent post at WebmasterWorld on the topic of Log Spam. Log spam is one of the oldest methods of link spam out there and it is often not discussed in forums (I know, compared to other methods, log spam is not discussed often). First, let me quote this excellent definition.
What is log spam? Oganisation setup a system to visit every domain (website) and 'request' their own domain (as if it were a page on the site).
What does log spam do? The result of this is in the logs (and therefore your stats page) in the request report (NOT to be confused with the referrer report) within the stats for the popularity of each of YOUR web pages you see links to the log-spammers pages.
Why do they do it? - Mainly to get links pointing to their site from stats pages, - Partly to encourage visits from those viewing stats pages. Sure, most stats pages are password accessed or have a 'noindex' meta tag, however there are many stats pages that are open or even have some PR.
Why is it bad? Log spam messes up your stats reports, e.g. you don't have a page called http://ebay/ but your stats report says that page has been viewed (requested) 100 times.
Now, in this thread, the thread creator said he caught Google log spamming. Its a fact that many well known Web sites do this, he said "Yahoo do it, ebay do it and lots of other companies do it." But why is Google doing it? Do no evil? Is this even true? Questions remain to be answered in the thread named Google in my 'log spam'.