I had problem building a working RCP application, so I put together the RCP Text Editor Example to see how to configure a RCP app.  If you want to build it yourself, instructions from the main RCP page at Eclipse site is below.  The page is actually a view into CVS, so I expect/hope the real main page will appear in the near future.

1. Download the RCP Runtime build from the Downloads page, and extract it to a folder such as c:\texteditor-example.
2. Download the RCP text editing component zip and extract it to the eclipse/plugins subfolder.
3. Download the texteditor plugin zip and extract it to the eclipse/plugins subfolder.
4. Replace the eclipse/configuration/config.ini file with the appropriate config file from the texteditor plugin. For example, on Windows, copy config_win32.ini file from org.eclipse.ui.examples.rcp.texteditor_1.0.0 plugin to eclipse/configuration and rename it to config.ini.
5. Run the eclipse executable: For example, on Windows, run eclipse.exe.

Step #4 is the key step while the rest are just downloading and expanding files.  After step #5, you should see a barebone text editor.  On Win32, the RCP text editor has disk footprint of 6MB and memory footprint of 28MB.  In comparison, NotePad2 has disk footprint of 540K and memory footprint of 4MB.  SWT apps I built before had substantially smaller footprints than the RCP app, but still significantly larger than native apps.

So what do we get for roughly ten times the footprints?  Plugins!  You can put together a fancy standalone IDE for Perl, Python, or PHP by packaging plugins from the EPIC, PyDev, or PHPEclipse projects with RCP.  You'll need to pour some sweats to glue things together because those projects are probably not fully RCP ready yet.

If you are not building an IDE, don't need IDE like level of extensibility, or there aren't many plugins useful to your product, you are probably better off skipping RCP and building on top of just SWT instead.