Per an email sent out to Mileage Plus and OnePass members:
We understand how important upgrades are to you. Currently, elite members of both programs enjoy unlimited upgrades on domestic flights on both United and Continental. This summer, you’ll be able to use your Regional and Systemwide upgrades on Continental flights (with a few limitations). And once we have completed our merger, you will enjoy your upgrade benefits across our worldwide network. Rest assured, we are working to make your upgrade experience consistent across our worldwide network, as quickly as possible.
This is great news. I suspect the “few limitations” are that there will be fare class restrictions on using systemwide upgrades, probably requiring a minimum of a “W” fare. Interestingly, at least domestically, Continental prioritizes “supported” upgrades over “complimentary” upgrades (meaning an upgrade using miles would clear ahead of a “complimentary” upgrade), so I’m curious to see which policy they go with.