Simple.
John Lewis vouchers (or vouchers for whomever your list is with - JL is the best option simply because of the vast range of stuff it carries).

We put some really nice pieces of furniture on our list - not expecting anyone to buy them, but so that if we were got vouchers (they're automatically given as an option for people looking at your list) it would be obvious what the vouchers were going towards. I'm fairly sure the vouchers last for quite a long time, so you could potentially set them aside for future need, such as a new washing machine/fridge/whatever.

I have to say that asking for money (whether towards honeymoon, home improvements or to "go towards the cost of the wedding" [and yes, I have seen that on a wedding invite/gift list thing before]) is unspeakably tacky, and should be avoided.
(The Empress it seems you CAN polish a turd!, Mon 7 Mar 2011, 16:47, Reply)