[jschioppa]

Possible fix if you're accessing an Exchange account via your 4S: I was having this same issue - reminders entered via Siri were not going to OF Inbox. I checked my iCloud Calendar as per the instructions above (thanks!) and noticed none of the Reminders I'd set were listed there either. I poked around a bit on my 4S and noticed that under Settings>Mail, Contacts, Calendars, all the way at the bottom there's a Reminders Default List which was set for Exchange. I was selected Reminders as the Default and Siri is now working like a charm.