:In my experience, at least in the modern garage punk scene, the previous role of cassettes is being replace by vinyl and mp3's; CDs as a whole are not an appropriate inheritor of cassette culture, and in fact aren't very popular in the community.  Mainly, I think it has to do with the cost of CDs in stores ($15/CD vs. $8 for an LP), the fact that they're not very durable (scratch, scratch), the cost of in-home CD players vs. turntables, and the old-style feel and sound (real or imagined) of records.  With the advent of mp3s and portable mp3 players, CDs aren't really necessary at all, even home-made ones.   

 :By way of example, most of the bands I come into contact with don't even release their music on CD at all.  The Spits, for example, released their first album on CD, and haven't made another release on CD since, even though their popularity has increased.  

 :As for postal service vs. cassettes, I don't think we're making a confusion here.  True, "Mail art" is important, but cassette trading was as much a matter of person to person interaction as it was long-distance interaction.  The "cassette" aspect of it is important because of the nature of cassettes; they can be recorded, re-recorded, copied directly, &c.;, and for most people of the time they were the only recordable media available (1/4 inch was expensive). 

 :But, these are just thoughts.  Since you didn't sign the comment, I'm not sure how old it is, but I'd be interested in your counter-point.