I can confirm this for Natty (daily image 2011-03-25).
I've did this (write in terminal): gksudo gedit /etc/apt/sources.list
Change this line: "deb cdrom:[Ubuntu 11.04 _Natty Narwhal_ - Alpha i386 (20110325)]/ natty main restricted" to: "deb cdrom:/ natty main restricted"
I can confirm this for Natty (daily image 2011-03-25).
I've did this (write in terminal): sources. list
gksudo gedit /etc/apt/
Change this line:
"deb cdrom:[Ubuntu 11.04 _Natty Narwhal_ - Alpha i386 (20110325)]/ natty main restricted"
to:
"deb cdrom:/ natty main restricted"