A lot has been said already about whether mono is safe, if it is from Microsoft, if it is a good language, if it is... well, all has been said I think. Personally I have no real objections against mono, but that it gives me another 100MB of download and that I have the feeling that it slows down my computer, especially on my eeepc. One more reason to not want a flame war... read this.
For Debian/Ubuntu based distros there is a special package (.deb) that conflicts with mono. This helps for users to be notified and/or prevent when mono will be installed. This does not exist for Arch (at least I did not find it). Also see the last part of this post!
As I think it is a nice double check, I created that package myself. The package is called nomono and If you want to download it you can find it here. You can install the package with:
pacman -U ./nomono-1.0-1-i686.pkg.tar.gz
Of course, this package is just a small helper tool. You should always read carfully through all the packages that you are about to install. This little package is just a help.
If you want to remove the package you can type:
pacman -R nomono
Just for your information: the package installs a small (5 line C program) called "mono" to remind you of the name of the package in case you want to remove it. Thus:
[me@myhost ~]$ mono > Mono installation is blocked by the nomono package.
Hope it helps
For Debian and Ubuntu based linux distros there is the mononono package. You can find it here. To be honest; the idea for creating this Arch package was borrowed from his (Tim Chase) package.