I was trying to put togehter a quick list of the placeholders on the default.master in WSSv3 and decided quickly that there were more than I expected.  Here's the list...

·         PlaceHolderAdditionalPageHead

·         PlaceHolderGlobalNavigation

·         PlaceHolderGlobalNavigationSiteMap

·         PlaceHolderSiteName

·         PlaceHolderSearchArea

·         PlaceHolderTopNavBar

·         PlaceHolderHorizontalNav

·         WSSDesignConsole

·         SPNavigation

·         PlaceHolderPageImage

·         PlaceHolderTitleLeftBorder

·         PlaceHolderTitleBreadcrumb

·         PlaceHolderPageTitleInTitleArea

·         PlaceHolderMiniConsole

·         PlaceHolderTitleRightMargin

·         PlaceHolderTitleAreaSeparator

·         PlaceHolderLeftNavBarDataSource

·         PlaceHolderCalendarNavigator

·         PlaceHolderLeftNavBarTop

·         PlaceHolderLeftNavBar

·         PlaceHolderLeftActions

·         PlaceHolderNavSpacer

·         PlaceHolderLeftNavBarBorder

·         PlaceHolderBodyLeftBorder

·         PlaceHolderPageDescription

·         PlaceHolderMain

·         PlaceHolderBodyRightMargin

·         PlaceHolderFormDigest

·         PlaceHolderUtilityContent

·         PlaceHolderBodyAreaClass

·         PlaceHolderTitleAreaClass

 I was going to write descriptions to help me figure out which ones were which but I ended up deciding the names were descriptive enough.  Then I realized that there were more than 30 placeholders ... I'm wondering why anyone would need a custom master with that many placeholders.  Wow.

[Andrew Connell pointed out that there's a page in the SDK which provides descriptions.  It's at http://msdn2.microsoft.com/en-us/library/ms467402.aspx.  Cool.  rlb 061215]