And for anyone curious - you can read ITS#3851 http://www.openldap.org/its/index.cgi/Development?id=3851 for the background on the problems in BerkeleyDB's memory allocator. While there was a fair amount of debate as to whether Jong's proposed solution was of any benefit, it was pretty clear that the existing code in BerkeleyDB could be improved. But the changes in the memory allocator may not visibly affect you if you're running a small-to-medium sized database. It's only apparent when the total volume of data is much larger than the BerkeleyDB cache, because that's the condition that leads to memory fragmentation, and it's the fragmentation that causes the allocator's performance to degrade.
To sum up - if you're already using BDB 4.5.20 with OpenLDAP 2.3 and the performance is acceptable, I wouldn't be in any hurry to upgrade to BDB 4.6. Releases 4.6.18 and 4.6.19 are incompatible with OpenLDAP 2.3. BDB 4.6.1 is a drop-in replacement for 4.5.20 though.
So currently the recommended BDB version to use with 2.3 is either 4.5.20 or 4.6.1 (or good old 4.2.52 + patches).
Ok, thanks.