So here's simple root package that is supposed to work for Arc. It's not tested as I don't have the device myself - just made it for my buddy who got the phone few hours ago and successfully unlocked.
I modified the original initrd to provide the root shell. Use it to copy su binary file + install Superuser.apk to achieve root.
This is only for people who understand what they are doing!
If you don't... just wait for some tutorials etc!
1. Unlock the bootloader
2. Download the boot.img http://www.mediafire.com/?il5sb1n45fw5dxa
3. fastboot flash boot boot.img
4. adb shell -> remount system rw -> push su into /system/bin
5. Install Superuser.apk