We have a new house and with it I've been upgrading all of my 20 year old TV and stereo equipment. My wife has been patient with my desires and wishes and we have worked to a compromise on the TV topic. Yes, we could get an HDTV, but it to be an inexpensive 27" TV (we picked a Samsung that is splendid for less than $1000). However, ironically, the month I decide I'm going to invest in that new McIntosh integrated amp I've sought for 20 years my wife decides we're going to track ALL of our expenses for that month to determine our monthly costs and create budgets. Doh! So we discussed it and agreed I would not share the cost of the amp (fortunately, she is not aware of the McIntosh value). But to be fair I'm going to reduce my other expenditures for the year. This is what I'm learning about marriage: it is all about comprimising to meet each other's needs and there is a balance which must be acheived. You both must respect each other's interests and allow each other to follow them. I agree as others have stated here that honesty is important. Admit that it IS an expensive piece of equipment, but be willing to make trade-offs in other areas, like holding off trading in that old Subaru for another year.
Before I was married a friend of mine gave me this advice: buy all your toys now because once you're married it won't be so easy to sneak them in the door.