If you are going to be utilising RODC in mixed environment (XP and 2003 ), then you need to download the Windows 2008 RODC Compatibility Pack. The Product Group and Dev Team, have been working exceedingly hard to get this released. It addresses the following issues as described by Product Group.
“To provide support for mixed mode operations (Win2003 & Win2008) in domains involving Win2008 DCs, all the features are made available for downlevel clients (XP and Win2003) also. However, since XP and Win2003 were not developed to support the additional checks and flags introduced in RODC, some of the features fail to work with downlevel clients are interacting with RODC. This is more apparent in Demilitarized (DMZ) networked branch offices . In this kind of setup, the clients are restricted access ONLY to RODCs and not other write-able DCs, outside the network.”
Source:→ Jane Lewis's Weblog