There are several mentions of roaming profiles in "man smb.conf" showing which configuration values you will be working with. However you will have other things to do, and it will depend on which version of Windows you use. You need to be using Samba as a Domain controller. For roaming profiles, if your users use Microsoft Outlook, you need to use an exchange server to hold the emails. The PST files will be gigabytes in size and take to long to download otherwise.
Look in Chapter 27 DESKTOP PROFILE MANAGEMENT of the Samba Howto and Reference Guide.
Check the using samba book. I think it has login script examples.