I just answered your support ticket with the same question. I will give the information here as well, so that it is available to all:
We recently changed the way the RadSplitter calculates its size on the client, in order to enhance the performance when there is a lot of content in the RadPanes. The problem was mainly in IE. The code that you refer to is changed on purpose and it is not a bug. In case you have problems with the new mechanism we have implemented in the splitter, please prepare and send us a simple running project, demonstrating the problem and we will fix it.
the Telerik team
Check out Telerik Trainer
, the state of the art learning tool for Telerik products.