There is an interesting theory that too many site wide links might do more harm than good to your Google rankings. Site wide links are unnatural and while a few might be acceptable (blogroll links for example) if your links are mainly site wides then it is likely you have been violating the Google guidelines by buying links or using some other unnatural method designed to manipulate your Google rankings.
On the flip side we know that MSN and Yahoo love site wide links and sometimes a couple are enough to get you top rankings in a few weeks.
What can a sneaky SEO do?
Assuming that you have more than one website and are using site wide links from one site to help another there is a fun technique called cloaking that you can use.
Simply set up your site wide links so that you only show them to MSN and Yahoo and not to Googlebot and your normal visitors. This way you can’t get banned from Google for cloaking and you get site wide links and top rankings in MSN and Yahoo.
The risk is that MSN and Yahoo might ban your site but this is unlikely as long as they don’t know that you own both the websites.
If you really don’t want to get banned by Yahoo or MSN you could show the sitewide links to your visitors as well and just remove them for Googlebot.