The first Fixpack for Service Pack 3 of Xcelsius 2008 is avaiable now through the SDN. You can download it here. The Fixpack is an incremental update.
Please be careful to choose the right version, as there are diffent versions for Xcelsius Present and Xcelsius Enterprise. (See the screenshot below)

As SAP currently has not provided any release notes or fixed issues document i currently can not provide any details about the fixpack. But i suppose it won't harm you if you stay up to date!

After successful installation you may check your Xcelsius to be version 5.3.1.0.