There's cool shopping cart migration web service Cart2Cart. It enables to automate the migration process.

Currently this service is running in beta mode thus all migrations are free of charge and everyone who will register on the website will get the account with 2 500 credits. This is about enough to migrate the content of middle-sized store.

Check it out

Great news from Cart2Cart - now you can also migrate to/from Magento.
Anytime soon support of CubeCart, osCMax and xtCommerce is expected.

This automated shopping cart migration service provides assisted migration if needed. It is still running in beta mode and all registered users get additional 2 500 credits for free.