The only shenanigans they could play that I could see would be to:
1. Take advantage of lack of patent protections in the Linux kernel and keep the patent licensing separate from the copyright licensing.
2. Take advantage of the fact that copyright controls can only be extended to derived works. If they figure out a way to use dtrace in the Linux kernel in a way that leaves it relatively unmodified then there is no effective way that it can be claimed as derived works and thus would be legal to keep licensed under the CDDL. It's something that was written for a different OS entirely and ported to several others, so only the linux-specific parts of dtrace would have to be licensed under the GPLv2.
This is similar to how Nvidia takes their Windows driver and shoves it into the Linux kernel with their 'GPL shim'.
But I don't know why people are so eager to assume that Oracle will pull crap like this. They have released many software patches and such as GPLv2. They are responsible for Btrfs and a lot of other work that was done to enhance the Linux kernel. As far as I can tell Oracle has shown no other signs or desire that they wish to make the Linux they ship be 'proprietary' in any substantial way.
I think most of this line of thinking stems from the common belief that Oracle sees Solaris as a potential competitor to Linux. I don't think that is Oracle's desire at all... I don't think they give a flying-f for Solaris or Sun hardware besides being a legacy OS and system that gets relatively minor updates time to time to keep the big players with solaris-specific applications happily puking huge gobs money at their feet. They Oracle may talk up a good game about Solaris, but I think that is just because big enterprise people tend to be nervous ninnies and need the reassurance that Oracle isn't planning to pull the rug out from underneath them.
Java was their primary concern as far as Sun acquisition goes. I think that Solaris was almost incidental. I don't think they care a whole lot one way or another as long as their is at least one very popular platform available to run their application stack that isn't owned by Microsoft.
We will see if what I say is true based on how Oracle decides to license it. If I am right then they will probably have the Linux-kernel portions be dual licensed and the userland parts may or may not stay under a CDDL-only... which is just fine because CDDL is a friendly free-software license.