This is a common question these days. Tradition states that this is a big No-No. But with the way weddings and etiquette have evolved over the last 20 years, and that living together before you're married is becoming more common-place, this may soon shake it's taboo status. Several years ago, including your registry information was even considered rude...but has become almost a standard addition. Here's a search that may give you some ideas on the subject: Invitation wording cash.

Most experts suggest using word of mouth to spread this information. Let your immediate family and bridal party members know that if they are asked by anyone where you and your fiance are registered, or gift suggestions, that money is your gift of choice.

If you do decide on including it on/in your invitation, here's a few suggestions for how to include it.

• Down play it. Make it as subtle as possible. Something like: Monetary Gifts Appreciated/Welcome.

• Make a joke out of it. Use a poem:
  
  Because at first we lived in sin
  We've got the sheets and a rubbish bin
  A gift from you would be swell
  But we'd prefer a donation to our Wishing Well!!
  Source
  
  Word of Caution: Make sure your guests are the type of people who will view this as funny and not insulting.

Hopefully this has given you some ideas of how to get the word out, but still be tactful.

__________________

I dreamed of a wedding of elaborate elegance,
A church filled with family and friends.
I asked him what kind of a wedding he wished for,
He said one that would make me his wife.
~Author Unknown