libcudart comes from the Cuda Toolkit
download from the Nvidia site.
From my initial poking around, I've not found an RPM that provides this yet, but these are early days. I'm not interested in BOINC per se - more coding applications direct for CUDA, but part of our journeys appear to be in parallel.
The installation of the toolkit is pretty trivial and simply involves running the script and editing your PATH and ld.so.conf.
I'm certainly running Cuda code on Fedora 11 x86_64 at the moment, but it is only by downloading and installing the drivers from Nvidia - not by using any of the RPMs that are available.
Hope this helps