Just for the record: It seems the new Debian Lenny release is close at hand.
Apparently a few packages for the lenny release already made it into the productive repositories, which causes them to be held back by apt-get upgrade:


apt-get dist-upgrade -V
Reading package lists... Done
Building dependency tree... Done
Calculating upgrade... Done
The following packages have been kept back:
libapache2-mod-php5 (... => 5.2.6.dfsg.1-1+lenny2)
libperl5.8 (... => 5.8.8-11.1+lenny1)
php-pear (... => 5.2.6.dfsg.1-1+lenny2)
...


apt-get dist-upgrade, which usually resolves this issue by upgrading the system to the most recent release does not work yet, since lenny was not released so far.