But, hmmm..I don't automatically open envelopes, especially envelopes with no return addresses, and especially envelopes with no return addresses that say "Please do not discard." 
There's actually a mailing our agent is about to do for us that we were originally talking about doing by postcards, but she felt that they should be done in an envelope with her agency return address.
Thanks for the insight, Jose and der Brucer.
Yes... And there are those like you who don't automatically open envelopes, so... You can't win 'em all.
Personally, I "prefer" an initial mailing done "envelope style" and then a follow-up with a postcard. First something to tell me about the project - and in a bit more detail than what can fit on a postcard (press release) - and then something I can pin to my bulletin or post on the door of my fridge.
And as DR DerBrucer alluded to... Some sort of identifying graphic/logo is a good way to tie those two pieces together.
*And just remember, all that junk mail that gets sent out... A 5% response is considered a "high" response rate! -But, then again, if you're sending out 100,000 flyers, I guess 5,000 new customers would be a good thing.