From one web site on grammar:

"Plural forms and the apostrophe (Bedford 36/Hodges')
A common error is to form the plural of a noun by adding "'s" to the singular form. Try to get out the habit of doing this. A trick for remembering that the apostrophe signifies the plural is that possessive means "having," and so the posessive form of the word "has" an apostrophe. How do you tell the difference in between plural and possessive? Check the meaning of the sentence. Does the noun simply refer to more than one thing? Or does the sentence mention something that belongs to the noun?

Although usage has changed in recent years, some handbooks call for an apostrophe in the plural forms of numbers, letters, and words used as words:

How many 1's do we have in the line?
We put x's on the incorrect answers.
The no's resounded loudly throughout the chamber."

http://www.meredith.edu/grammar/plural.htm

From another website on grammar:

"An apostrophe is used in plurals in the following very special cases:

in the plurals of single letters:
There are only three s's in `Christmases'.
Mind your p's and q's.
(Even here, the capital letter would not need the apostrophe.)

in the plurals of abbreviations:
We have several pg's [paying guests].
We have received four cheques and two IOU's.
(But IOUs is common and accepted, and the usual plural of CD is CDs).
Most symbols for units such as lb (pounds) and cm (centimetres) do not strictly have plural forms.


in the plurals of numerals:
This house was built in the 1930's.
(But 1930s is preferable).

As an alternative spelling, for clarity, of the plurals of a very few short words:
We went to several society do's last year.
While out with his third wife he met both of his ex's.
I've had yes's for coffee from four people.
But in each case, dos, exes, yesses would be acceptable. The usual plural of no is noes."

http://www.askoxford.com/asktheexperts/faq/aboutspelling/pizza

Even the experts do not agree on the issue; thus, as Peace pointed out, both forms ("noes" and "np's", "1960s" and "1960's") are acceptable.

A.