Kevin Frie, development lead for core parts of the CLR recently posted about what CLR updates that come in 3.5 SP1....

• NGen infrastructure rewrite: the new infrastructure uses less memory, produces less fragmented NGen images with much better locality, and does so in dramatically less time. What this means to you: Installing or servicing an NGen image is much faster, and cold startup time of your NGen’ed code is better.
• Framework Startup Performance Improvements: The framework is now better optimized for startup.
• Better OS citizenship: We’ve modified NGen to produce images that are ASLR capable, in an effort to decrease potential security attack surface area.
• Better 32-bit code quality: The x86 JIT has dramatically improved inlining heuristics that result in generally better code quality, and, in particular, much lower “cost of abstraction”.