The PEAR blog has pointed out some issues that the latest Ubuntu release (Karmic) can cause with a typical PEAR installation:

Be aware that the initial release of Ubuntu Karmic contains a bug that affects PHP and PEAR, whose fix came a tad too late to make the initial release. The bug is fixed, and will be included in upcoming updates from Ubuntu. [...] The bug causes some zlib functions to be unavailable to PHP, and the Archive_Tar code will silently fail due to this.

Until the update gets pushed out into the updated releases, you can use the "-Z" option on any package installs or upgrades you might do in your PEAR installation.