Hello,
Let me explain in a little more detail what semiologic is all about.
Bear in mind I am not a semiologic affiliate or have any financial interest in promoting their product. With that in mind, what I have discovered is that if you are looking to build a large number of websites/blogs semiologic is very well suited for this.
The structure of a semiologic website is based on the "Silo Structure." The silo structure is based on static pages on which you create the information that will sell your prospects. The power of WordPress blog is in creating posts. Posts are kept on a separate page called "blog." (according to Matt Cutts this is the preferred term for your blog.)
Here is some more information on silo structures and a really good image of what it looks like:
http://www.seo2020.com/lsi/silo-structured-website.html
This way you have the ability to get indexed and drive traffic to your site without having your sales pages compromised.
Regarding search engine optimization the semiologic theme comes with an SEO plug-in that was created by them. They also include over 65 plug-ins that take care of things like on page 301 redirects, Google analytics, etc..
Rather than explain everything here in this post it is perhaps better that you visit this link that will explain the entire silo structure of a semiologic theme.
http://www.semiologic.com/resources/seo/silo/
To give you a better idea of the power of back links and the semiologic theme take a look at this website.
http://www.danielmolano.com/
(By the way this is not my website) this website uses the current stable version of the semiologic theme. There is a new theme called semiologic reloaded which is significantly more flexible and better looking than what they have had before. The stable version of the semiologic theme will be released once all the bugs in WordPress 2.8 have been worked out. Which should be any day now.
Back to my example, this website is currently ranked on page 1 for the key word "business consulting." If you will Google the term you will find that there are exactly 8,350,000 plus exact search results for this keyword. This keyword has approximately 49,500 global searches per month/27,100 local search volume. To say the least it's a highly competitive keyword. You can also check the "inURL" and "intitle" and you will see that the key word is also will SEO'd
If you do a little more digging you will find that this website was created on November 2008. (Approximately 8 months old) if you do a little more digging you will also discover that the website has 1456 back links. My purpose in giving you all this information is to show you the power of using a semiologic theme along with some good search engine optimization tactics. The site was built from what I can see without the use of any black hat strategies. It's a white hat site from what I can see.
Now, some of the detractors of the semiologic theme would say that this is not a "pretty sight" and to those folks I say "I really don't care." In developing my websites I am not looking to be the prettiest site I am looking to have a site that gets traffic and make sales.
Here is an additional link that explains all the widgets that come with a semiologic theme. By the way, you can get the basic semiologic theme and most of the plug-ins for free at their website. The other advantage I have found using the semiologic theme is that it comes with three different layouts and approximately 32 different skins. So each website does not have to look the same as any other that you own. It also allows you to "clone" your websites. Once you have created your first site you can easily clone it to your next website so that all your plug-ins and site structure is the same. This saves you a great deal of work in setting up your website.
(By the way the developer of semiologic Dennis de Bernardy, has been very heavily involved in the upgrade of WordPress 2.8 and so understands the WordPress platform very well.)
I hope this has helped you understand a little more about what I was talking about. Good luck