I have never encountered the usage of 'DH' and 'DD' until I came to Usenet. I've interpreted them to mean 'Dear Husband' and 'Dear Daughter'. Is this usage regional? I seem to get the impression it may be used mainly in the South, but I may be wrong. It also seems to be confined to women - I've never seen a husband refer to his wife as DW, nor have I seen it used with sons.
For the first time, Google yielded nothing when I did a search on this. Thanks for any background on the use of these terms.