The problem is that Subversion and ClearCase really use two completely different ways of looking at your repository. Here are some basic issues:
* Subversion revisions: Because Subversion revisions the whole repository instead of individual files, people use the repository revision as a sort of pseudo tag. You might talk about a piece of software that was built from revision 23323.
ClearCase has no concept of change lists, so you'll lose all that information. To get it, you'll have to create special labels for each and every revision in Subversion and apply them to ClearCase. That'll take forever to accomplish.
* Subversion branches are merely directories designated as branches, and Subversion duplicates the entire directory tree in a branch. ClearCase creates an actual branch, and it uses sparse branching. If out of 10,000 files only one is changed on a branch, only that file is branched. Duplicating this means branching all files for all branches.
Another problem is that there's no easy way to look at the repository and decide what is a branch of what file. In ClearCase, you have a single instance of a file and that instance contains all versions and branches. In Subversion, tags and branches don't necessarily have a logical connection to the file they're from. You can parse the log, but that's going to be difficult.
* Subversion has attributes that have some very specific meaning. The biggie is svn:external. If you used that, you're sunk. There's really no easy way to duplicate this on ClearCase.
The best thing you can do is just lock your Subversion repository as "read only" and bring only the tips of the active software projects over to ClearCase.
Even if somehow you could move your entire history over to ClearCase, it might end up being fairly meaningless. There's no way you can guarantee you can checkout and rebuild, and have it be accurate. In the end, you'll need to keep your Subversion repository up anyway for a while.
By the way, why are you making this switch? I've seem plenty of people go from ClearCase to Subversion, but never the other way around. Your developers are going to throw a fit because it'll take a lot of effort for them to learn the ways of ClearCase, and many of the tools they use might not work.