Memory or storage?
I am getting it too. Droid X, Gingerbread stock. It happens sometimes when installing a program and often when updating. Sometimes it will fail, but when I try a few times, it will work.
I have more than 1 GB available in internal storage and 8 GB on the SD card. But my RAM is constrained. Although the specs for my phone shows 512 MB of RAM, I am showing about 384 MB. Applications seem to hold onto RAM even after they are closed. Killing apps frees some up, but it quickly refills.
I am going to guess that a memory management function is trying to efficiently use most of the available memory, releasing it only when a more active program wants it. But there is a problem with the installation and/or updating process that causes it to not understand that memory could be available and throws the error.
Total speculation, of course.