Last week when talking to a customer about their new MOSS 2007 environment I was asked whether it was possible to adjust the top level navigation on all My Sites.
In general it is possible to adjust the top level navigation by going to the Site Settings page.
Under Look and Feel you can select Top link bar. Here you can add a new link. Unfortunately doing so in your My Site would mean that you would have to repeat that for every user's My Site.
In order to adjust the top level navigation in all My Sites you should go the Central Administration and to the SharedServices. Here you will find the link Personalization site links under User Profiles and My Sites.
You can add new links here that will be added to the top level navigation between the My Home link and the My Profile link.
You can also target these links, so you could for instance put a link in there that would point to "My Business Unit". This can be realized by adding a link with the title "My Business Unit" for each of the business unit homepages. These links would then be targeted to audiences that are generated for each business unit. This way a user would only see his or her own business unit link displayed in the My Site.
Of course if you work for two separate business units and if you are in more than one business unit audience the link would be displayed twice, pointing to the two different business unit homepages. So it might be smarter to use the name of the business unit as the title for the link. Oh well, this was just an example.. ;-)