The SharePoint Developer Team just blogged to clear any confusion around the future of Sandboxed Solutions:

While developing sandboxed solutions that contain only declarative markup and JavaScript — which we call no-code sandboxed solutions (NCSS) — is still viable, we have deprecated the use of custom managed code within the sandboxed solution. We have introduced the new SharePoint app model as a replacement to those scenarios that required the use of managed code.

Read more here …