After playing around with Sidewiki for a few days I haven't changed my opinion, but I have made a few interesting observations as to how Sidewiki works and how people might manipulate it. Just like Google bombing, in an attempt to influence the top Google result for a specific search term, I suspect people will quickly master the art of Sidewiki bombing as they attempt to get their comment at the top of the Sidewiki comments on key pages.
So far it seems that most of the Sidewiki comments on the web are on blog posts actually talking about Sidewiki - which is understandable. It also supports the argument that Sidewiki is hijacking the debate on websites. As I've looked around I've discovered a few pages, such as the front page of the White House, which have links to relevant blog posts in them. At first I thought the site administrator must have found a way to flood the Sidewiki bar with an RSS feed, which would be a brilliant way to stifle comments on controversial websites, but then I remember that this is Google's doing. Sidewiki aims to include links to "relevant posts from blogs and other sources", as well as comments from Sidewiki users. I'm not sure if this is enabled for every site, as so far I've only found a handful of examples.
Searching through Sidewiki comments is an interesting exercise in itself. There are already a few Sidewiki monitoring tools that let you view the comments on a site. You can also do a Google search for "site:www.google.com/sidewiki/ +searchterm" but the results don't seem to be complete. You can easily subscribe to an RSS feed for a user's comments, but not the comments on a site. There is an RSS workaround using the following and substituting the URL you want to follow; http://www.google.com/sidewiki/feeds/entries/webpage/http:%2F%2Fwww.google.com%2F/full
Whether Google planned it or not, the RSS feature seems an effective way to push other comments off the front Sidewiki page. When I looked at it on the White House site, the first comment that wasn't a link to a blog post was buried on page 3, even though that particular comment was more recent that the stories linked to in the comments before it. New comments appeared at the top of the Sidewiki page, but they obviously don't stay there long if old new stories occupy the first few pages.
Depending on exactly where Google sources the "relevant posts from blogs and other sources", this feature might be an effective way to bomb Sidewiki. If you can link to your site with enough blog posts, and mark them as useful, it should push other comments further down the list. As long as you can push everyone else off the first page of Sidewiki comments, you can consider Sidewiki jammed on your site. It may rely on you having access to prominent blogs, such as those listed on Google News, which would obviously make it harder for your average man on the street to pull it off. If prominent bloggers wanted to, they could probably launch a Sidewiki denial of service attack on a site by repeated linking to it and pushing all other comments further down the list (thus my experiment).
Of course the ability for users to mark a comment as useful or not is open to abuse. Take a look at the lone anti-Israeli Sidewiki comment on http://www.goisrael.con/tourism_eng - I'm sure the Israeli tourism board was unaware it was there but it might be gone by the time you get there because by drawing attention to it I may have cause it to be marked as abuse. Perhaps if enough people mark is as not useful it will disappear, but if enough people mark it as useful then it might stay even if others mark it as abuse. Such sites are going to turn into a battleground, just like controversial pages on Wikipedia. Just like Wikipedia, Google will eventually be forced to introduce safeguards against Sidewiki abuse.