Can a phone manufacturer's custom interface be changed?
Your best bet would be to go with either LauncherPro: https://market.android.com/details?id=com.fede.launcher my personal favorite, or ADW: https://market.android.com/details?id=org.adw.launcher

Both provide customization to the dock (set of icons at the bottom), number of homescreens, transitions, app drawer effects, as well as custom themes and widgets. Note: there are free versions of both, but the paid versions add more features.

While you can cover up some of what Samsung has done, things like the Notification bar and lockscreen aren't affected by these apps. In some cases rooting and/or installing custom ROMs would be required to manipulate them.