A while ago I wrote a blog post to explain how to monitor subscription status.  You can read it here: http://blogs.msdn.com/lukaszp/archive/2005/12/30/monitoring-subscription-status-also-calling-rs-soap-methods-from-inside-reports.aspx

Today, I updated the Subscriptions List report contained in that entry and published to enhance its functionality.

I also added 3 reports that will help you see which subscriptions use which delivery extension.

You can get the reports here:

http://cid-e5eac755bcb2e151.skydrive.live.com/self.aspx/.Public/New%20Subscription%20Reports

New Subscription List Report: http://cid-e5eac755bcb2e151.skydrive.live.com/self.aspx/.Public/New%20Subscription%20Reports/SubscriptionsList.rdl

Extension used by subscriptions: http://cid-e5eac755bcb2e151.skydrive.live.com/self.aspx/.Public/New%20Subscription%20Reports/SubscriptionsByExtension.rdl

Sub report - Standard Subscriptions: http://cid-e5eac755bcb2e151.skydrive.live.com/self.aspx/.Public/New%20Subscription%20Reports/StandardSubscriptionsExtension.rdl

Sub report - Data Driven Subscriptions: http://cid-e5eac755bcb2e151.skydrive.live.com/self.aspx/.Public/New%20Subscription%20Reports/DataDrivenSubscriptionExtension.rdl

After you download all these files to your machine, create a new report project in Visual Studio/BIDS and add the RDLs. 

Then click the preview tab to see what they do.

The reports are quite interesting in and of themselves.  Between the reports, I use the following features of Reporting Servies:

XML data extension, expression based connection strings, internal parameters with validation logic (value substitution), data set filters, data set sort, subreports, custom data set fields, code functions in reports, show/hide/visibility control, SOAP web services, charts, etc.

Take care and good luck,

-Lukasz