Hello Alex Brown,
Thank you for using our services.
It is possible to create your own controls, which work with services. In fact we do use WebServices for getting content items for example in the Sitefinity back end. I can point you to two blog posts for using WCF services and WebServices in Sitefinity:
Enable loading of CMS pages through an AJAX Web Service
Use the Content Manager API in a Web Service
As for the login control - we do use the standard ASP.NET Login Control. You can either override this control, or create your own which uses Forms Authentication. The source for the login control can be found in ~/Sitefinity/UserControls/Login/LoginControl.ascx.
Please, feel free to get back to us if you have more questions.
the Telerik team