Hmmm, some quick testing shows that both a normal request and a request made from updating the Top Sites preview images, will probably yield the same User Agent string in the final release of Safari 4.

However, with the current public beta, Top Sites gives me an additional "Public Beta" in "Version/4 Public Beta":

Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10_5_6; en-us) AppleWebKit/528.16 (KHTML, like Gecko) Version/4 Public Beta Safari/528.16

...whereas actually looking at the page, yields a ".0" suffix, but no "Public Beta" in "Version 4.0":

Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10_5_6; en-us) AppleWebKit/528.16 (KHTML, like Gecko) Version/4.0 Safari/528.16

But I assume that this difference will be taken out once Safari 4 is released, and indeed I think that is to be preferred (though I could understand that a webmaster might want to know if a site is listed in Top Sites).

(I do have the developer tools enabled; when changing the User Agent using menu Develop, User Agent then the selected User Agent is only used for normal browsing, not for Top Sites.)