This week's book giveaway is in the General Computing forum. We're giving away four copies of Arduino in Action and have Martin Evans, Joshua Noble, and Jordan Hochenbaum on-line! See this thread for details.
If you explicitly put such a field into a serializable class, then you can use it to provide versioning for serialized data files containing this class. Class versions with the same SerialVersionUID have compatible serialized data, by definition, even if the class changes.
But if this doesn't matter to you, then just ignore the warning.
For your information: in Eclipse you can change the importance of compile errors/warnings to your taste by Window - Preferences - Java - Compiler - Errors/Warnings. Specific this one can be found in the group called "Potential programming problems".