In our current extension, we have code like the following to ensure
that code is run on the UI thread.  The code below is an over-
simplification of the actual algorithm, just trying to show use
pattern.  Decision was made to use this pattern based on
http://www.mozilla.org/projects/xpcom/Proxies.html and ease of coding.

In FF2, nsIProxyObjectManager.h defines appropriate macros for being
able to find the proxy manager (NS_XPCOMPROXY_CONTRACTID) or also
gives visibility to NS_GetProxyForObject().

--- example code begin

int MyObject::notify( unsigned long cmd ) {
  nsresult rv;
  nsCOMPtr<nsIProxyObjectManager> spProxyManager;
  spProxyManager = do_GetService(NS_XPCOMPROXY_CONTRACTID, &rv);

  nsCOMPtr<IMyObject> prox;
  spProxyManager->GetProxyForObject(
    NS_UI_THREAD_EVENTQ,
    NS_GET_IID(IMyObject),
    (IMyObject*)this,
    PROXY_ASYNC | PROXY_ALWAYS,
    (void**)&prox );

  prox->processCommand( cmd );

--- example code end

In FF3, now it appears that these are trying to be "hidden" (use is
discouraged?)
- NS_XPCOMPROXY_CONTRACTID now defined in nsXPCOMCIDInternal.h
- NS_GetProxyForObject is now hidden behind #ifdef
MOZILLA_INTERNAL_API

In FF3, is there a new preferred way for communicating from worker
threads to the main thread?  If so, is there a sample extension that
is already using this model?

This is not hidden/discouraged. This is the header file we use for contracts
that are not frozen. Go ahead and use it with the ordinary cautions about
that it may change in future releases and your code may stop working.

This is correct: you cannot link against this method. However, you can use
regular XPCOM calls to do the work, e.g.

  nsCOMPtr<nsIProxyObjectManager> proxyObjMgr =
    do_GetService(NS_XPCOMPROXY_CONTRACTID);
  if (!proxyObjMgr)
    return NS_ERROR_FAILURE;

  nsCOMPtr<nsIFoo> proxyObject;
  nsresult rv = proxyObjMgr->
    GetProxyForObject(targetThread, NS_GET_IID(nsIFoo),
                      object, nsIProxyObjectManager::INVOKE_ASYNC,
                      getter_AddRefs(proxyObject));

--BDS

Would the following also work?

class MyEvent : public nsRunnable {
  private:
    unsigned long m_cmd;
    nsCOMPtr<IMyObject> m_spMyObject;

  public:
    MyEvent( unsigned long cmd, IMyObject *pObj )
      :m_cmd(cmd),
       m_spMyObject(pObj) {}
    NS_IMETHOD Run() { m_spMyObject->processCommand(m_cmd); }

int MyObject::notify( unsigned long cmd ) {
  MyEvent *pMyEvent = new MyEvent( cmd, this );
  NS_DispatchToMainThread(pMyEvent);