BSD license allows you to re-use fixes of others (assuming they also release their fix under BSD), but it does not compel them them to reveal their fix if they distribute only a binary.

GPLv2 requires modified public distributions to also be under GPLv2, so you could both see the fix/change and use it (in the sense that you won't be infringing on copyright).

GPLv3 adds patent protection, so someone adding a fix/change that they have also patented explicitly grants a license for that patent.

I think GPLv3 is the most comprehensive way to keep fixes/changes of others available for reuse.

If you use the GPL you will want to add a lisp-specific preamble such as is found in the LLGPL (http://opensource.franz.com/preamble.html).

Don't forget that if you use the GPLv3 you won't be able to distribute your software with the OLPC, since it has anti-theft measures which put signed code at an advantage to unsigned.

If I recall correctly, the Apache license has some kind of clause to the effect that all contributors automatically grant you all the same rights available under the license, meaning that by contributing, they acknowledge the license and give you permission to make use of their contribution. There is also some anti-patent stuff.

http://www.apache.org/licenses/LICENSE-2.0

OTOH, it's a big, long license, and perhaps the BSD license would be simpler, which is always an advantage.

I'm for a BSD type of license.