nope that is what unemployment insurance is for
If MS have any sense they won't lay off 1000's people because there probably aren't enough running Yahoo to start with these days and they really don't want to make it crash. Just hopefully they aren't going to convert it all to .NET and Win Vista Server Edition or we might need more than a relief fund!
I am not a Microsoft employee, and cannot speak for the company, however...
As a former long-time employee of Microsoft, Visio, and then Microsoft again after Microsoft purchased Visio, I have some experience in this situation. YAHOO employees do not need to consider this a dire situation. In fact, it's likely to be financially beneficial.
Microsoft has acquired many companies and is very well organized and professional about the process. The company is clear and communicative about everything. Employees in each category are treated fairly and equitably. The people who are working in "core" areas will continue to work uninterrupted. Those who are in redundant roles, such as administration or payroll and so on, will, most likely, be given opportunities to interview for other jobs. Any transition will be cushioned. In fact, I turned down a few job offers and ended up accepting a very good job at Microsoft.
The only down-side is the fact that the culture encourages work-a-holics and is led by executives who demand success. Employees tend to be very smart, and nerds are not uncommon.
Most employees at YAHOO will benefit should the acquisition go forward. I believe that this will have great benefit to both companies.
Don't worry about it. Grease Monkey is hiring!
Plenty of jobs for everyone!