RadTextBox currently does not support rounded corners due to controls rendering restrictions. You can get rounded corners if you replace the RadTextBox controls with TextBox, and add RadInputManager to your page to keep the RadTextBox functionality. RadFormDecorator will then add rounded corners to the TextBoxes.
We are currently preparing compatibility sheet with a purpose to set a unified styling for all input type controls, no matter whether they will be with or without rounded corners. An issue that slows us is the backwards compatibility, that said we need to make sure that we will not break any customers' projects with any possible changes.
To further clarify - We cannot state that we will achieve cross-browser consistency, mainly due to specific browser restrictions. The idea is to get a consistent page on a cross-control stage, not on a cross-browser. So, if you have a page and open it in IE7 all controls should look the same, yet without rounded corners for example. In IE9 on the other hand all controls should also look the same and they should also have rounded corners.
These issues will be addressed as soon as Q3 2011.
Yet, it is not clear from your question whether you want to have the controls with rounded corners or without, or you just want to have them consistent? In case you need further assistance, could you please open a support ticket with your requirements.
the Telerik team