I would love to join a block exchange. We just finished one here at a local group and will be exchanging them at our Christmas party on the 15th.
How we did ours was we gave each participant a fat eighth of our focus fabric and let them know what size block we wanted back. Each person then chose fabrics from their own stach to make the blocks from the focus fabrics. Of the 47 in our group 18 participated. Our rules are must join and have fabric to everyone by Feb 1 and have your colpleted quilt for "Show n Tell" at our July meeting the following. Example we exchange our finished blocks on Dec 15 2007 and will complete them by July 2008. This is the second year of this exchange. It sure was fun to see what everyone did with their blocks. Sky's the limit on what you can do; quilts with interesting settings and/or sashings, potholders, placemats, table runners, pillows, table toppers and even window valences and curtains.
Hope this gives you some ideas.