When looking at a nurture campaign stream, the program should show how many leads have reached a certain piece of content within the nurture stream. I saw this feature when we were testing Eloqua (pre-Marketo-deployment) and I don't know why Marketo wouldn't offer the same type of convenient high-level visibility.
If you change the stream view from Content to Leads, you can see the total number of leads who received the content and the number of leads who received it in the last cast. You can also click the link to see the actual names. So I believe we have what you are asking for.
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