You can run Debian on the sun4u (SPARC 64) architectures using the released stable distribution. Just use the SPARC architecture with the supplied 64-bit Sun4u kernels. You'll be running with a 64-bit kernel, but all applications will be 32-bit (although you won't notice the difference). Most of the Debian SPARC distribution was compiled on Sun4u machines.
For more information, errata, and status, see the Debian SPARC port pages.