Just to clarify:
No, you cannot access it anyway you want. Specifically, you are not allowed to access the tiles directly.
But ... as pointed out in Rex's blog that Derek links to, it is OK to use Google Maps as long as you connect to Google Maps with their interface/API. Thus it can be legal to use Google Maps together with the ArcGIS API for Flex.
While Google doesn't have a Silverlight API, they do have an ActionScript API. So for Flex applications, you are not limited to the "Google Static Maps API" (with the size limite), but can instead use their "Google Maps API for Flash" for maps of any size.
Disclaimer: this is my interpretation and I have no legal expertise. See http://code.google.com/apis/maps/terms.html for the google legalese.