The answer Ancestry.com is providing for now is "maybe" and here's where they explain it:
1. Go to "Your DNA Home Page"
2. Click on "View Results"
3. Click on "See Full Results"
4. Place your cursor over your Uncertain percentage and a text box with their explanation will appear.
This explanation can also be found in the AncestryDNA FAQs. See #4 under "Interpreting My Results."
I have 7% Uncertain ethnicity so I'm very interested in knowing what it represents as well.