Approval Request Comments in Visualforce Email Template.

ApprovalRequestComments.html

<apex:component controller="ApprovalRequestCommentsController" access="global">
    <apex:attribute name="relatedToId" assignTo="{!targetObjectId}" type="String" description="ID of the record whose last approval comments to retrieve"/>
    <apex:outputText value="{!comments}"/>
</apex:component>

ApprovalRequestCommentsController.cls

/**
 * As of Winter '14 release, email templates used with approval processes can
 * include {!ApprovalRequest.field_name} merge fields.
 * For example, {!ApprovalRequest.Comments} merge field returns the most recently
 * entered comment in emails for an approval step.
 * 
 * However, the merge fields do not work in visualforce email templates.
 * Thankfully, Pradeep on Developer Forums came up with a solution to use
 * a visualforce component in the template backed by controller that queries
 * the approval step information.
 * 
 * This class represents the controller in this workaround solution.
 * Also see ApprovalRequestComments visualforce component.
 * 
 * Inspired by https://developer.salesforce.com/forums/ForumsMain?id=906F00000008xjUIAQ 
 * 
 * http://docs.releasenotes.salesforce.com/en-us/winter14/release-notes/rn_186_forcecom_process_approval_comment_merge_fields.htm
 */
public class ApprovalRequestCommentsController {

    // ID of the record whose most recent approval process comments to retrieve
    public ID targetObjectId { get; set; }
    
    // The most recent approval process comments
    // Could show in visualforce email template, for example
    public String comments {
        get {
            if ( comments == null ) {
            	ProcessInstanceStep lastStep = getLastApprovalStep();
            	comments = ( lastStep != null ) ? lastStep.comments : '';
            }
            return comments;
        }
        private set;
    }
    
    public ApprovalRequestCommentsController() {}
    
    // Queries the most recent approval process step for the target record
    private ProcessInstanceStep getLastApprovalStep() {
        List<ProcessInstanceStep> steps = new List<ProcessInstanceStep>([
            SELECT
            	Comments
            FROM
            	ProcessInstanceStep
            WHERE
            	ProcessInstance.TargetObjectId = :targetObjectId
            ORDER BY
            	SystemModStamp DESC
            LIMIT
            	1
        ]);
        return ( steps.size() > 0 ) ? steps[0] : null;
    }
    
}

ApprovalRequestCommentsControllerTest.cls

@IsTest
private class ApprovalRequestCommentsControllerTest {

    @IsTest
    private static void test_get_comments() {

        // As of 12/31/2014, Salesforce does not provide us ability
        // to create ProcessInstance or ProcessInstanceStep records,
        // so there is no way to test this *correctly* without actually
        // submitting a record through an actual approve process.
        // However, this does get 100% code coverage =/
        
        ApprovalRequestCommentsController contr = new ApprovalRequestCommentsController();
        System.debug( contr.comments );

    }
    
}

Email Template.html

<!--
   Note, for the comments made by the final approver, you must explicitly send this email template
   as an email alert in the Final Approval and/or Final Rejection actions. This is because the
   email template for "Approval Assignment Email Template" field on the Approval Process is only sent
   when the record is assigned to the next approver. For final approver/rejection step, there is no next approver,
   hence the need for a Final Approval/Rejection email alert.
 -->
<messaging:emailTemplate subject="Opportunity Approved"
    recipientType="User"
    relatedToType="Opportunity">
<messaging:htmlEmailBody>

Comments: <c:ApprovalRequestComments relatedToId="{!relatedTo.id}"/>

</messaging:htmlEmailBody>
</messaging:emailTemplate>

Thank you so much for this!

This is great Douglas. Visual Force Email template is needed for CPQ Quote lines in Approval Email templates.