Interestingly, the latest build of KDE 4 (4.3.90 aka 4.4 RC1) no longer supports the underscore character in host names.
While this was allowed in previous KDE4 versions, the KDE team have removed support for the underscore as “STD3 requires all DNS domain names to be limited to Letters, Digits and Hyphen.”
Here are two examples of bugs that have been filed and subsequently closed with a ‘WONTFIX’ resolution: 220500 222291
So, any of you sysadmins out there who have the audacity to have hosts (or DNS aliases/addresses) on your network with the underscore character in them, you’ll no longer be able to connect to those hosts using KDE4 apps like KRDC (Remote Desktop) or the Konqueror web browser.