Google wants to index all the existing public profiles of Google users, so it created an index of sitemaps and placed the address at google.com/robots.txt. Since the Sitemap protocol has been adopted by all major search engines, creating a sitemap is the easiest way to inform search engines about a large number of web pages from your site that don't have backlinks.
The sitemaps include about 150,000 Google profiles and this seems to be the number of people who took the time to create a profile in Google Maps, Google Reader and the other services that integrated the unified profiles. Since Google didn't promote this feature, the number of people that create a profile will increase once it becomes more visible.
As Jérôme Flipo noticed, Gmail started to link to the profiles pages of your contacts, probably only for Google Talk friends.
To create or edit a profile, visit this page and enter as much or as little as you want. "A Google profile is simply how you present yourself on Google products to other Google users. With a Google profile, you can easily share your web content on one central location. You can include, for example, links to your blog, online photos, and other profiles such as Facebook, LinkedIn, and more," explains Google.