Jan 26 charmod draft, section 3.5

The reference processing model says "The specification MUST NOT arbitrarily 
restrict the range of characters that can be used, which must cover all 
Unicode code points from 0 to 0x10FFFF inclusive." and later goes on to say 
that XMl 1.0 complies with the model. XML 1.0 in fact arbitrarily forbids 
the use of many Unicode characters. -Tim