There is a temporary solution if you don't mind going back to Pages '09 with your new Pages 5 document. Pages 5.0 gives you a choice export your Pages 5 document as "Pages '09". Export as "Pages '09" (make sure to label it as created for merging) and then open in previous version of Pages. Updating to Pages 5 did not destroy your previous 4.3 version. It's in your apps!
Now! If you were using a Numbers spreadsheet for your data source, and have also upgraded to the newer version on Numbers, you can follow the same workflow. You will have to export your spreadsheet as Numbers '09 and use that as your merge source. SO you will have to continue to open and work with your Pages '09 and Numbers '09 documents to do your merges until Apples provides us an update containing a mail merge workflow. Ugly I know, but it will tie you over. Apple never leaves you hangin' forever.