libmono-dev has been removed in mono 2.10. It is replaced by libmono-2.0-dev
Following packages must update their bdeps when mono 2.10 hits unstable:
dd-list $(grep-dctrl -FBuild-Depends -FBuild-Depends-Indep libmono-dev -sPackage /var/lib/apt/lists/*Sources | sed -e "s/Package: //"|sort -u) Mirco Bauer <meebey@debian.org> gluezilla (U) gnome-desktop-sharp2 (U) gnome-sharp2 (U) gnome-subtitles (U) gtk-sharp2 (U) mod-mono (U) mono-addins (U) mono-debugger (U) monodevelop (U) monodevelop-vala (U) smartirc4net (U) webkit-sharp (U) xsp (U) Debian CLI Applications Team <pkg-cli-apps-team@lists.alioth.debian.org> blam gnome-subtitles gshare monodevelop monodevelop-vala Debian CLI Libraries Team <pkg-cli-libs-team@lists.alioth.debian.org> gnome-desktop-sharp2 gnome-sharp2 gsf-sharp gtk-sharp2 log4net mono-addins ndesk-dbus nemerle smartirc4net taoframework webkit-sharp Debian LibreOffice Maintainers <debian-openoffice@lists.debian.org> libreoffice Debian Mono Group <pkg-mono-group@lists.alioth.debian.org> gluezilla mod-mono mono-debugger xsp Eric Dorland <eric@debian.org> lat Sebastian Dröge <slomo@debian.org> gluezilla (U) gnome-desktop-sharp2 (U) gnome-sharp2 (U) gshare (U) gtk-sharp2 (U) log4net (U) mono-addins (U) ndesk-dbus (U) nemerle (U) taoframework (U) Rene Engelhard <rene@debian.org> libreoffice (U) Evgeni Golov <evgeni@debian.org> libindicate (U) Sam Hocevar <sho@debian.org> taoframework (U) Dylan R. E. Moonfire <debian@mfgames.com> log4net (U) mod-mono (U) taoframework (U) xsp (U) Francesco Namuri <francesco@namuri.it> drapes Carlos Martín Nieto <carlos@cmartin.tk> blam (U) Siegfried-Angel Gevatter Pujals <rainct@ubuntu.com> mistelix Jo Shields <directhex@apebox.org> gluezilla (U) gnome-desktop-sharp2 (U) mod-mono (U) monodevelop (U) monodevelop-vala (U) webkit-sharp (U) xsp (U) Jose Carlos Garcia Sogo <jsogo@debian.org> gsf-sharp (U) The Ayatana Packagers <pkg-ayatana-devel@lists.alioth.debian.org> libindicate Tiago Bortoletto Vaz <tiago@debian.org> gnome-subtitles (U)