According to Google Offers Named OpenIDs blog entry I should be able to create a named Google profile. However, going to http://www.google.com/profiles redirects me to my Google+ profile with 21 digits in the url and I don't see any option to change it.
If you have a Google+ account, Google Profiles redirects automatically to G+ and you are not able anymore to create/set a profile. Even if you have/had a named profile, it redirects to the 21-digit Google+ profile for now. Google said it will allow Google+ named profiles in the future, but for now this option is not available.
As an alternative, there's a third party service GPlus.to which gives a short name of form
gplus.to/<username> and redirects to your Google+ profile