In response to Ivy, with respect: if the letters are uppercase, then "dotting the i" has no meaning; neither does crossing the upper-case "t" have any. No dot in the first instance, no crossing in the second.

The first example is correct. Merriam-Webster's Dictionary of English Usage prefers it, using the example of "mind your p's and q's."

An apostrophe is not used when making a word plural, and, increasingly, also not used when making a number plural. But in the case of an individual letter the apostrophe is still required, particularly when its absence would cause confusion. In this case, "is" is a word, so the absence of an apostrophe could give momentary misdirection to the reader.

The site referenced by Anonymous coward says that both are correct, as porsche notes. But if not wrong—and it is—the double quotation mark construction is, at the very least, inelegant.

First, if you think that “most of us will agree” on anything you must be new to this site.

Second—and this really should have been first—there can be no hard rule as to what preposition follows “different.” Consider the following:

“Sheila thought serving pizza for breakfast would be different.”

Is a comparative is implied? Not really. For all we know Sheila usually serves calamari in the AM. No comparative is present. Consider:

“Sheila thought serving pizza for breakfast would be different from the usual eggs.”

A comparison has been made, therefore a comparative word—from—is used.

But in another instance “to” might be correct:

“Sheila thought it would be different to serve pizza for breakfast.”

Whatever: Sheila is no chef. Last, this:

“Sheila thought nothing could be more different than to serve pizza for breakfast.”

Except, of course, calamari.

The point is that “different” may be followed by various prepositions depending on context.

English is like that. It’s different.