The market went down, but Google is doing ok. They got very good results according to their last report that was announced this week. After this report Google's stock went up, Yahoo stock - same. Bottom line, I don't think that they will change the pay rate (at least not until the end of this year).
markets down means companies will spend less on their advertising budgets... Which will affect your adsense earnings.
Stock market is just a mirror that reflects what is going on in global economy. If we end up with negative global economical growth companies will definitely spend less on advertising. However IT companies are rather unaffected by the crisis, thereby not all keywords will be losing their value. However in some time, keywords related to finance/banking/mortgage etc will most likely loose on their pay rate.