This will make it easier for systems administrators to deploy Silverlight into enterprise environments. As from last week , Silverlight is available as an optional update for Windows XP and Windows Server 2003 via the Windows Server Update Services tool (and, inevitably, Microsoft Update, since WSUS relies on Microsoft Update as its source for updates).
Many customers have requested help with deploying Silverlight internally, and this change will hopefully be welcomed by enterprises who want to distribute Silverlight within their organizations using their established management tools. By adding it as an optional update, enterprises can control the roll-out of Silverlight within their organizations and schedule its installation as a background task so that the perceptible impact is minimal, and ensure that end-users can view Silverlight content without requiring administrative rights to install the plug-in.
I'm really glad this is now available: the need for administrator rights to install a web plug-in is not unique to Silverlight, of course, but it's been raised a number of times as a potential blocker to enterprise adoption. Having this available will solve that problem and make it far easier for enterprise administrators to control deployment of Silverlight to their users.