And a now, word on the log.php maintenance script that comes with Magento 1.4 which has one major shortcoming.

It doesn’t maintain your dataflow_batch_* tables, said tables have been seen hiding in the wild at over a Gigabyte in size.

php -f ./shell/log.php help gives you a listing of the various options

php -f ./shell/log.php status gives you a listing of the log tables, how many rows and the size of storage space contained in the tables and indexes.

php -f ./shell/log.php clean --days 15 will clear all but the last 15 days of log contents (minimum 1 day)

php -f ./shell/log.php clean clears everthing per your settings in system setups under log maintenance