Hi Jeff,
First, I would like to note that the way you wrote the PROC MIXED code corresponds to RCBD with subjects as blocks. If that is the case, then the standard errors for all tretament levels are computed using the same formula:
se[y_bar]=SQRT{(var1+var2)/n},
where var1 and var2 are, respectively, a between-subject variance (block variation) and a MS(Error), and n is a number of subjects.