United Airlines has, somewhat quietly, launched a sale for mileage-based upgrades into BusinessFirst on most routes the carrier flies. Passengers can save 20% off both the miles and any cash co-pay required on travel between the US-49 or Canada and Europe, Japan, North Asia, Middle East, Central/Southern Africa or Southern South America. This promotion applies only for upgrades requested online through July 28, 2012. Even if the request is waitlisted due to no award inventory available at the time of request the discount will apply.
The notable destinations which are excluded from this promotion are Hong Kong, Ho Chi Minh City, Bangkok and Taipei. All destinations in North America, the Caribbean and northern South America are also excluded, though these are not routes marketed with BusinessFirst service so less surprise there.
The discounts are nice but it is still a stretch to consider cash+miles upgrades a decent value.