Synchronizing your machine keys is a great way to provide a single-sign-on experience when using ASP.NET Forms Authentication. This tells ASP.NET a specific key to use when encrypting/decrypting the ticket, and when multiple servers or applications use the same keys, they will accept the ticket generated by the inital login.
When I tried adding a 2.0 application to the mix, it did not work. The solution took some digging, but was pretty simple. ASP.NET 2.0 allows you to specify a decryption attribute in the machineKey config element. ASP.NET 1.x did not let you specify it - Triple DES was used by default. However, ASP.NET 2.0 uses DES by default - specifying DES as the decryption algorithm resolved the issue.