Você está executando uma distribuição mista (stable / testing / etc) ou usando repositórios secundários?
sudo apt-get install php-pear
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following extra packages will be installed:
libonig2 libqdbm14 php5-cli php5-common
Suggested packages:
php5-dev
Recommended packages:
php5-suhosin
The following NEW packages will be installed:
libonig2 libqdbm14 php-pear php5-cli php5-common
0 upgraded, 5 newly installed, 0 to remove and 0 not upgraded.
Need to get 4,167 kB of archives.
After this operation, 12.3 MB of additional disk space will be used.
Do you want to continue [Y/n]? n
Abort.
cat /etc/debian_version
6.0.6
Se este for o caso, você deve fazer o downgrade de todas as versões do pacote não-mainstream e tentar novamente.