[moskie]

Wikipedia's article on Ohm actually covers this!

http://en.wikipedia.org/wiki/Ohm#Ohm_symbol

"Unicode encodes the symbol as U+2126 Ω ohm sign, distinct from Greek omega among letterlike symbols, but it is only included for backwards compatibility and the Greek uppercase omega character U+03A9 Ω greek capital letter omega (HTML: Ω Ω) is preferred."

And from the Unicode Standards doc that is the source for that section:

"Greek Letters as Symbols: The use of Greek letters for mathematical variables and operators is well established. Characters from the Greek block may be used for these symbols.

For compatibility purposes, a few Greek letters are separately encoded as symbols in other character blocks. Examples include U+00B5 µ n the Latin-1 Supplement character block and U+2126 Ω in the Letterlike Symbols character block. The ohm sign is canonically equivalent to the capital omega, and normalization would remove any distinction. Its use is therefore discouraged in favor of capital omega. The same equivalence does not exist between micro sign and mu, and use of either character as micro sign is common; for Greek text, only the mu should be used."

[afandian]

Is there a separate symbol for A?