1. It's two spaces after a mono-spaced font like Courier, and one space after a proportionately spaced font like Times or Bookman or Palatino. In a monospaced typeface every character has exactly the same width; an upper-case W is just as wide as as lower-case e. In a proportionately spaced typeface every character is different in terms of the space it takes, and the period character (like the ! and the ?) already has extra space, so you only need one space.
2. This isn't "new"; it's been true for more than a hundred years.
3. People associate it with typewriters because for a very long time (until the 1980s or so) most typewriters used mono-spaced faces, so you needed two periods.
Anyone who obsesses over this and isn't a typesetter is a newt. It's dead easy to change, and an editor who flagged every single extra space is not using the tools built into MSWord to do his or her job. If it's a pattern throughout the MS. flag a single instance and explain. Flagging every instance means that the file will become bloated and is more likely to be corrupted. It's entirely unprofessional.
* If you can, use one space after a period or other terminal punctuation when you're using mono-spaced faces and two spaces after a period or other terminal punctuation when you're using proportionately spaced faces.
* If your editor or agent has a particular fetish for one-or-two spaces, or submission instructions specify something, do a search-and-replace before you submit. Do the search and replace on a copy of the file, and make two passes, in case you've got an instance where you mistakenly have three spaces. It's pretty common to do that.
