Interesting topic...
I personally don't think gift vouchers are good for the web hosting market. It is different when it's for a book store such as Ottakar's [Boy do I miss that store... no good book store here in France], or a coffee shop such as Starbucks or a chocolatier such as Thorntons.
With a book store - You have the voucher, you go in, and choose from a range of hundreds of books. And there is bound to be one that you like.
With a coffee shop - You have the voucher, you go in, and buy coffee or buy something else that they sell. Gift vouchers do not limit you to one item, it only limits you to what that shop sells [Or at least this is how it works on the U.K.]. And even if you do not like coffee, they are bound to have something you like.
But really, a gift voucher for a web hosting company? It seems a bit far fetched. I can just imagine now an old lady giving her grandson a gift voucher for web hosting!
It's not that I don't think it's professional, as many companies do offer gift vouchers. However, I cannot really see it happening in the hosting industry.
Gift vouchers are usually used as a present when you have no idea what to get the person. The first thing that comes into your mind is a gift voucher. And to be totally honest, I cannot see someone giving another person a gift voucher for a web hosting company as a present. You tell them the URL of that company, you don't give them a gift voucher for web hosting from that company.
However... offering your clients a gift voucher from another company as a way to bring in more clients is another idea. And an idea that I think could work a lot better than selling gift vouchers for your own company.