The following Terminal command will revert the Help Viewer window to its "not always floating" 10.4 behavior:

defaults write com.apple.helpviewer NormalWindow -bool true

Additionally, execute the following (bash) commands as an administrator in Terminal to have access to the application via Command-Tab or its Dock icon:

i=/System/Library/CoreServices/Help\ Viewer.app/Contents/Info.plist
sudo defaults write "${i%.plist}" LSUIElement 0
sudo chmod 644 "$i"

Sources:

• Non-floating window command from Ars OpenForum.
• Command-Tab and Dock icon from macosxhints.

[robg adds: This hint makes the second half of this previous hint (which required hacking the Help Viewer executable code to change the window behavior) irrelevant. There's still some good info in the previous hint if you'd like to not only gain a dock icon for Help Viewer, but also change the window's behavior (by editing its .nib file). For most people, though, I suspect the simple Terminal command in this hint will be sufficient. I went through the Help Viewer binary files in the 10.5 release, and I honestly don't recall seeing the NormalWindow string anywhere, so this may be new in 10.5.3 (or I just overlooked it with the earlier release).]