I think the decline in the 1990s had much more to do with the collapse of the Soviet Union, which removed the Cold War rationale for aid. A long literature confirms that a lot of aid is motivated by non-development foreign policy concerns--the Cold War, the "Global War on Terror", the U.S. war on drugs, policies of maintaining ties with former colonies, or with oil exporters, etc. I'd guess that *that* kind of aid is less vulnerable going forward. Aid that is more developmental in motivation is probably more vulnerable. I'd guess that the "donor darlings" like Uganda and Tanzania are therefore most vulnerable to cuts. At http://blogs.cgdev.org/globaldevelopment/2008/10/history_says_financial_crisis.php, I show what has happened to aid after past rich-country financial crises: down every time.