Be careful with this property. If you set it true in the list definition, there is no easy way to set it back to false after the list has been populated.
SPList object has two properties AllowContentTypes which is ReadOnly and ContentTypesEnabled, which you cannot set to true if the list is created as stated above.
If you do an export of the list and look at the Manifest.xml you can see that there's a property called Flags which has this number combo without any sense. After some reverse engineering i found a point that this string begins with number "137" if the DisallowContentTypes is set to true.
Do not touch the database. :)
If you want to change that, you'll need to modify the Manifest.xml and package it back to a new cab and do an import with it. (Remove the existing list before import). I cannot provide any guarantees that will not break anything but it seems to work.
There's also one interesting property in the ImportSettings class. Setting RetainObjectIdentity to True will keep the GUID's as in the source list. Nice little feature if you have data in other lists that depends on the guids of this lists items.
Enjoy