So I have like 30,000 frequent flier miles with American Airlines and in a few weeks, care of my employer, will be traveling US Airways for a business trip. Since American Airlines is now owned by US Airways does that mean that frequent flier miles earned on US Airways will count towards American? Or will I probably just lose them since I don't have a frequent file mile acct # with US Airways?
If past merger information is any indication of what will happen you can take a look at recent mergers of United and Continental.
The accounts of frequent flyers have been merged into one without the loss of miles. So until the announcement from AA and USAir come out you could operate under the assumption that your miles will remain intact.
At the moment you cannot earn miles on US Airways in your American Airlines AAdvantage account:
No-one knows for sure what will ultimately happen with the programs - possibly even American and US Airways at this stage - although it seems fairly sure from their official publicity that there will be a merged frequent flyer account. The publicity also states that it's likely to be with Oneworld, so I wouldn't be surprised if US Airways' Dividend Miles program is merged into American's AAdvantage, presumably gaining earning/spending miles privileges on Oneworld, and losing similar status with Star Alliance. (Related Economist article)
If you know for sure you will fly US Airways soon, and it's a non-trivial number of miles you'll earn, it might be worth you signing up for the US Airways program now, earning the miles, and hoping they give you the opportunity to merge them into your AAdvantage account later. I certainly wouldn't worry that your US Airways miles will suddenly become worthless - that would be a huge PR misstep, and possibly have legal and accounting issues for the new company. They are highly likely to allow you to do something with them, or at least give a grace period for spending them.
Additional: Some more information, some of it supposition, in this NY Times article, which seems to be based on an interview with the two CEOs, and is therefore presumably reliable: