## Ruby 1.9

    num = "#{?b.ord-?a.ord}"
    one = num.to_i
    limit = ?U.ord-?A.ord
    loop {
    	puts num
    	num = num.gsub(/(?<digit>.)\k<digit>*/) { $&.size.to_s + $+ }
    	break if (limit -= one) == one - one
    }

A golfed version:

    k="#{?b.ord-?a.ord}"
    j=k.to_i
    i=?U.ord-?A.ord
    loop{puts k
    k=k.gsub(/(?<d>.)\k<d>*/){$&.size.to_s+$+}
    (i-=j)==j-j&&exit}