Although there is nothing about it on Foxit's Web site, the company confirmed on the phone that this is a bug-fix release.
On May 20, Secunia issued an advisory that pointed out what it called a "highly critical" bug in the prior version. Secunia expected a fix from Foxit in an upcoming version of the software, however, as of 12:15 p.m. EDT on Sunday May 25th (roughly two days after the software was released), Secunia still lists the bug as unpatched.
Update May 26, 2008: Secunia has confirmed that the bug they wrote about is fixed in this release.
If you use the portable version of Foxit, as I suggested back on May 6, then simply download the Zip file again and delete the older version. If you use the normally installed edition of Foxit, then you can check for updates with Help -> Check for Updates Now. The new version will show up as "Foxit Reader 2.3.2008.2923 Upgrade" if you are running the previous version. If you don't use Foxit at all, give it a try.
The Foxit servers appear to be swamped. I experienced multiple failures both checking for updates from within the program and trying to download the Zip file.