You are definitely not alone. Our wedding is in December 2009 and here's where things stand:
Sent out "Save the Date" email back in January and mailed the invitations out in the middle of May asking for RSVP online by mid September. Only 1/3 third of the guests acknowledged receiving the invitations and 1/5 of the guests have logged onto the web site for details. Since they are spread out all over the globe, I am emailing the rest of the guests to confirm invitations were delivered.
It is a hassle with all the communication but most guests seem to be pretty nonchalant at this point. However, as RSVP deadline approaches, there should be more response. Keeping my fingers crossed.