Robert Scoble pointed me to One Versus Two Spaces After a Period, a report by John S. Rhodes.  The reported enumerated several issues and concluded with the advice that only one space should follow the period.  I disagree for following reasons.

Proportional versus monospace fonts – while proportional fonts allows the eye to more easily identify the beginning of a new sentence, width of a single space in proportional fonts are generally narrower than the uppercase character used to start the new sentence.

Compuert versus typewriter fonts – Period character for computer fonts are much smaller than period character for typewriter.  For small computer fonts, typically used to display news articles or blog posts, a period is just a single pixel.  Width of the period is also much narrower than on typewriter, making the period difficult to see.

River of blank spots – River of blank spots do not appear in left justified text typically used for news articles.

Large font means larger spaces – I do agree that double spaces should not be used for text displayed in large fonts.  Sometimes we have no control over the font size, but we can usually assume a reasonable range of font sizes users are expected to view the content in and take appropriate actions.

Ultimately, I just like double spaces better than a single because I find it less disorienting in a long paragraph.  I can also more easily locate a paragraph.