Don't call me Shirley!

So I'm working through some example AJAX stuff that I've found in the documentation and I'm trying to apply it to a custom object I've made:

<apex:page controller="MyController" tabStyle="Person__c">
    <apex:pageBlock title="Hello {!$User.FirstName}!">
        You are displaying People from the app.
        Click a person's name to view his or her details.
    </apex:pageBlock>
    <apex:pageBlock title="People">
        <apex:form >
            <apex:dataTable value="{!people}" var="aPerson" width="100%">
                <apex:column >
                    <apex:facet name="header">
                        <b>Name</b>
                    </apex:facet>
                    <apex:commandLink action="{!invokeService}" value="{!aPerson.FullName__c}" rerender="resultPanel">
                        <apex:param name="id" value="{!aPerson.id}"/>
                    </apex:commandLink>
                </apex:column>
                <apex:column >
                    <apex:facet name="header">
                        <b>Number</b>
                    </apex:facet>{!aPerson.PrimaryContactNo__c}
                </apex:column>
            </apex:dataTable>  
        </apex:form>
    </apex:pageBlock>
    <apex:outputPanel id="detail">
        <apex:detail subject="{!$CurrentPage.parameters.cid}" relatedList="false" title="false"/>
    </apex:outputPanel>
</apex:page>

With MyController looking like this:

public class MyController {
    public PageReference invokeService() {
        Id id = System.currentPageReference().getParameters().get('id');
        //result = [SELECT Name FROM Person__c WHERE Id=:id].Name;
        result = [SELECT DateOfBirth__c FROM Person__c WHERE Id=:id];
        return null;
    }
    public List<Person__c> getPeople() {
        return [SELECT Id, FullName__c, PrimaryContactNo__c FROM Person__c
            ORDER BY LastModifiedDate DESC LIMIT 10];
    }
}

Thing is, I keep getting this error:

Error: MyController Compile Error: Variable does not exist: result at line 5 column 9

I've tried adding ".DateOfBirth__c" to the end of the braced SOQL to no avail as the original code had ".Name" at the end...

Any idea as to what I'm missing?

Cheers,

Dom