I've noticed this same behavior (where a '#' character in a URL causes the page not to display). It has only happened since I used the new 5.x series alpha; all my previous 4.280 (and 4.275 for Linux) installs are working great.
Can people with the '#' bug please list what version of Iron they are using, what platform, and any extensions they are using?
For the record, I have IETab, Google Mail Checker Plus, and XMarks on all my Windows based systems (Win7 x64 and x32), and just the latter two on my Linux based systems (Ubuntu 9.04 x64, 9.10 x32).
Again, this is only happening on my Win7 x64 system with Iron 5.x Alpha.