My OC side was alarmed when suddenly, my Problems view in Eclipse was filled with warnings on my XML files. Each of my XML files had a warning with it, and that little yellow exclamation mark on the side:
No grammar constraints (DTD or XML schema) detected for the document

So how do you get rid of it? Go to Window > Preferences > XML > XML Files > Validation then set Indicate when no grammar is specified to Ignore. Click on Apply.

Clean up your project (Project > Clean).

If the problem doesn't go away, you may need to re-validate the XML files. Right click on the file then choose Validate from the popup menu. You can also right click on the folder (such as your res folder) and validate from there.