I am not sure I clearly understand your question. But one suggestion when it comes to deciding how and where to deploy your application, espeically if you want to plan for scalability, is to consider Windows Azure: http://www.microsoft.com/windowsazure/[^] Azure supports both ASP.NET and PHP.

Which binding type are you using with WCF? I believe you should be using basicHttpBinding.

Out of curiosity, are you upgrading to 2010 MOSS or Foundation?

While this doesn't necessarily help with SharePoint 2007, have you looked a the improvements to support mobile devices in SharePoint 2010? From the SharePoint Blog, the #5 improvement for SharePoint 2010 Sites: http://blogs.msdn.com/b/sharepoint/archive/2009/10/19/sharepoint-2010.aspx[^] SharePoint Mobile Access – We both improved the experience for mobile web browsers and are introducing a new SharePoint Workspace Mobile client so you can take Office content from SharePoint offline on a Windows Mobile device. These clients let you navigate lists and libraries, search content and people and even view and edit Office content within the Office Web App experience running on a mobile browser.

Based on your bullet points above, I am guessing you know the users logged into the application. In the event of a needed application or server shutdown, could you send an email notification to logged in users? Another option ... are you using a master pages? You could consider placing a control in the header or footer which could display the status of an application or server shutdown.