I am also a member at bookcrossing.com and they just came out with a really nifty idea: Reusable envelopes.  They are Tyvek w/velco (can take them also, but since it's velcro & not glue it's easy to slit them open, then reseal.

I thought others here might be intriqued.  Also have a neat book logo on the outside. :)

http://secure.bookcrossing.com/product.asp?s=ch3sri6397&strParents=&CAT_ID=&P_ID=197

Pretty nifty idea.   I wonder if they come open if the books are not in them tightly, but can move around.   I may order some and find out.

So anyone along the way can open these up? I thik this would just make it easier for people to steal the books in transit or they may just fall out with all the tossing in shipment.

Well, you can put a piece of tape over the seal, which makes it no harder to get in that currently.