Originally from ticket #564.
There are a couple of ways to achive this customization. The simplest way is to edit the excm.resx or excm.en-US.resx file located in the App_GlobalResources directory of your IIS site. The .resx file is an XML file containing the resource string. You can then search for the wording you want to edit and replace it with your desired content. An IIS reset will be required.
The downside of this method is that the .resx file may get stomped when you apply and future ExCM patches or when an 'stsadm -o copyappbincontent' command is executed. Be sure to make a copy of the .resx file after make the change. If you want a more permenant solution then you may...
1. Create a new folder under "C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\TEMPLATE\LAYOUTS"
2. Navigate to "C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\TEMPLATE\LAYOUTS\SPSolutions\ExCM" and then copy the "SignIn.aspx" page.
3. Copy the page to the folder created in Step 1
4. Modify the copied aspx page to include your custom content
5. Go to Central Admin -> Manage Web Apps -> Select Web App -> Authentication Providers -> Click Default (or your applicable zone) -> and set the custom sign-in URL.