I was in town today and my local Game store are advertising 'Get A Free Nintendo DSi When You Trade In 12 Nintendo DS Games'. According to the assistant, the offer is basically trade in Any 2 Nintendo DS Games (Working UK Games with box & Instructions) for £25, hence trade in 12 games and get a Nintendo DSi (£149.99).
The offer is supposed to be on for the next 3 weeks (presumably over Easter holidays) and if you did not want a Disc, You should be able to receive store credit for the games you trade in.
EDIT: There seems to be some store discretion in regards with the offer. Some stores will allow you £24.99 on a gift card for 2 DS Games (or £150 for 12 games) while others may insist on trading in 2 DS Games to get a new DS game for £4.99.
It's probably best to check with your local store but If you do have to buy something, you should be able to return it (unused) within 28 days for or a refund to a gift card.
This is on til the 22nd April!!!!!!