A page on the Steam FAQ has revealed plans to introduce coupons - virtual discount vouchers which will sit in users' Steam inventories alongside other tradeable items.
No official announcement has come from Valve yet, but the support page for the coupons lists a number of details about the scheme.
Coupons will be limited to one per purchase, and will have specified games for which they are valid, although it appears that this will not be limited to one game per coupon - indicating that publishers could perhaps issue them for their entire catalogue. Coupons can also be applied to items which have an existing discount or sale price in place, unless specified otherwise.
Coupons will have expiry dates, indicated in a detailed description in the Steam Inventory. The coupons will also be tradeable between users, swappable for any of the other items which reside in the inventory - meaning that they can be used to buy in-game items and vice versa.
The program is a continuation of the Steam Wallet initiative which has seen the introduction of pre-paid funding and player trading mechanisms.