The biggest trends in direct mail envelopes are to make them look personal and then to make them bumpy. AT&T just sent out a marketing piece which was a pillow envelope. Inside was a piece of styrofoam to hold the pillow and a magnet with the marketing info. Well the styrofoam was lame (or laime?) but my wife could not resist opening it. A friend of mine sends a letter telling about his company and what his product can do for you with a nice ($5 or better). Or at least he would send you the outside of the pen. In the letter he would tell you that he would drop by to give you the other half.

Of course, all of these ideas are to a very targetted list.
Richard