Hi
Openldap isn't _that_ bad! Lots of work has gone into it to make it faster and more scaleable, and it absolutely the de facto standard these days thus is has far better support and compatibility. In terms of Zimbra, I would have thought OpenLDAP is the least of it's scalability problems - there are lots of other big opensource chunks in there too, although perhaps the flexibility of multimaster would be a big help to clustering.
FDS is a great bit of software but is has a small dev team improving what is essentially the old Netscape DS code - ie _very_ old code. Sun ONE DS nee iPlanet branched a long time ago from this code and has been developed, tested and used in very very large environments. As this is going to be opensourced I would be inclined to do as someone suggested on this thread and concentrate on making Zimbra generically compatible with any LDAP backend rather than specifically port such a quirky software. The main problem with FDS is the archane building mechanism and dependencies - I have gone through the pain in the past of 'porting' it to Solaris and IIRC debian as an exercise, at which point I switched to different software and never looked back - it's a pretty horrible experience.
WRT to SASL, Zimbra has added an extra auth mechanism into the Cyrus tree, apart from being a bit out of date there's no reason it can't be used with FDS or any other software, after all that's the whole point of SASL
I am certainly very interested in contributing to this effort, there are some great DS products out there that would fit the bill, I'd be interested in trying to use SunONE. The biggest problem at the moment is the lack of svn tree. There's no point developing at the 'source code' level of zimbra at the moment because you can't track their source, every release you have to start again or go through the pain of diffing your old tree and carefully patching the new, making sure all your changes are what you intended in the new tree.
Zimbra has and is doing a great thing for opensource messaging/groupware, but combination of it's licence, dual product and lack of source tree is starting to make it look less like opensource and more like a commercial proprietary product that allows people to view it's source code - big difference.