I recently had to "resend" a notification which Forms sent out and had to build the URL out of the Forms DB.
You can read the Forms SQL database and see the list of pending approvals and formulate a link from there. It's kind of crude but the only way I know how to do it w/o using the solution Blake suggested.
The DB table [cf_bp_instance_approvers] contains all pending actions in the format
[cf_bp_instance_approvers]
[approver_id] = Not 100% on this one but I believe it's an auto incremented # for tracking which approval was captured and saved into the archive.
[resume_id] = The session id, needed for the link
[user_snapshot_id] = The forms application associates an ID to a user. Mapped by the [cf_user_snapshot] table.
So, if you wanted to send out a mass mailer you could run a sql query:
SELECT 'http://lf/forms/form/approval/' + [resume_id] AS URL, [email]
FROM [LFFORMS_SERVER].[dbo].[cf_bp_instance_approvers]
LEFT JOIN LFFORMS_SERVER.dbo.[cf_user_snapshot]
ON [LFFORMS_SERVER].[dbo].[cf_bp_instance_approvers].[user_snapshot_id] = [LFFORMS_SERVER].[dbo].[cf_user_snapshot].[id]
Assuming your forms URL is http://lf/forms and your database name is [LFFORMS_SERVER] this will spit out the URL to take the user right into their task and their email address.
Task URL email
http://rochsvr139/forms/form/approval/51a5522d-bc0b-4fb6-b63b-bee7a6217e2f CHunter@mailserver.com
You could group this by email address to send out one email w/ multiple tasks.
The downside is you lose the specific information you may have configured in your notification in the business modeler. You could continue to dig through the Forms SQL DB to find the specific stuff but it gets hairy pretty quickly. Also, with Forms 10 coming up, and this reading right out of the DB, there's no guarantee this will work with that version.
But as far as I know, this is the closest I can get to sending out one email to a Forms users notifying them of all their pending tasks if they don't want to log into the Forms application and check their inbox.