Marco,
Well, GPL seems to meet most of my requirements for licensing issues, so I'll prefer using it.
This means that you can't _use_ BSD code and distribute the result.
From section 2 of the GPL:
b) You must cause any work that you distribute or publish, that in whole or in part contains or is derived from the Program or any part thereof, to be licensed as a whole at no charge to all third parties under the terms of this License.
This means that you'll have to find a GPL'd compiler to target, and you'll only be able to use GPL'd libraries. The fact the lisp leaves you no option but to "link" directly to other code makes the GPL far more restrictive than it is for compile-edit-debug languages.
In fact, I'm really far away from open source licensing issues and can't eloborate precisely at this time. Also I've thought about LLGPL, probably it will be really better than GPL for Lisp development.
As far as I understand, LLGPL is also permitted on cl.net?