How can I use the Chinese IME with a Dvorak layout?

Save as .reg file and run (as admin on Win7, I guess... or just use regedit to make the change manually):

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Keyboard Layouts\E00E0804]
"Layout File"="kbddv.dll"

This changes the keyboard layout that ‘MS Pinyin IME 3.0’ for Chinese (Simplified) delegates to to two-handed Dvorak. This does give me ‘你’ for ‘lg’ on a QWERTY keyboard on XP.

If you want a different IME or layout variant you'll have to change it a bit. If Windows 7's IME has a new ID number you might have to look through the other subkeys in ‘Keyboard Layouts’ in regedit to find which E00... code corresponds to the new IME.

For some background see this post by Michael Kaplan (MSKLC author).