Best info I've seen on the subject. From an ALPA email last night (emphasis in last paragraph added by me):

Several pilots have contacted the MEC office or their LEC representatives with concerns about the recently posted CAL System Bid. The UAL MEC System Schedule Committee responds below:

It is important to remember that under the s-CO contract, bids are only required to be issued twice a year. The company can elect to issue System Bids more often and can also offer Adjustment Bids. Partially due to their infrequent nature, these bids must attempt to address the company’s anticipated manpower needs for an entire year. Unlike the s-UA process we are more familiar with, these bids also account for surpluses and anticipated shuffling of flying. Because they try to encompass such a broad range of issues, the bids look very large compared to what s-UA pilots are used to seeing. As we are all well aware, this type of long range manpower planning is not very accurate. Many of the bids in this system bid may be cancelled or changed if a new CAL System Bid is issued in January 2013 or by an Adjustment Bid. This bid just provides a snapshot and a method for the company to get started getting pilots where they think they will be needed.

S-CO System Bid 13-08 (projected out to August 2013) contains 451 vacancies. 71 are for planned retirements and 28 are for shifting flying between domiciles with no backfills. Most of these vacancies are in order to avoid repeating the understaffing issues they experienced on the s-CO side this summer. In addition, some of these new vacancies are the first tranche of ramping up for increased needs in a potential JCBA and an acknowledgment that many of the s-UA pilots currently flying at s-CO will elect to return to their jobs at s-UA once recalls begin at s-UA. In addition, they also account for some of the pilots who will be unavailable for flying due to contract implementation duties for management and ALPA. Finally, some of these vacancies are a small net increase due to 787 and 777 (one aircraft) deliveries over the number aircraft that come out of service on the s-CO side between now and August 2013.

On the s-UA side, since the company can wait until closer in to issue vacancies, we will see any required bids in smaller tranches. Also, largely due to the TPA but also for cost reasons, the company wants to avoid surpluses as the flying moves around the s-UA system. Any bids we see will account for retirements and adjustments in flying. Just like at s-CO, our staffing is driven by the summer peak months. Also, just like s-CO, the company projects a slight decrease in hours for next summer due to economic issues for both companies. There is no plan for a net increase in aircraft and hours at s-UA but there is a plan to replace all but 15 757s with 737 aircraft over the next two years beginning next spring. This will result in an increased training need and many other transitional issues that still have to be worked out. In addition, any increased need will be offset by the above mentioned s-UA pilots now flying at s-CO who return to s-UA. The SSC believes the company should begin furlough recalls immediately (and the issue has been repeatedly presented between the highest levels of ALPA and the company) but due to their block hour forecast, the company believes it can hold off on recalls until Q1 2013.

In short, while the raw numbers in the s-CO System Bid understandably cause concern to s-UA pilots, they must be taken into perspective. It is not a sign that flying is shifting from one side to the other. As the time comes for the company to address needs on the s-UA side, we will see bids as necessary.