MacBook Pro 15.4" -- Current Model, Released in October 2011: "...out of all Mac computers, the MacBook Pro sees the biggest drops in price. The...current gen MacBook Pro has seen more aggressive discounts with up to $299 off after just three months."
MacBook Air 11" Core i5 -- Current Model, Released in July 2011: "...it hit an unprecedented low of $850, which is $29 lower than the best deal on the previous-gen MacBook Air (released in October 2010)...we saw generous discounts on the 11" Core i5 MacBook Air from multiple retailers including Best Buy and Amazon."
iPad -- Current Model (new iPad), released in March 2012: : "...although we would have predicted waiting at least three months for a modest discount on the recently-released new iPad, and then at least six to 12 months for more aggressive deals, we surprisingly saw a deal last week that took a full $40 off; it was extremely short lived, lasting barely a half hour"
See the full details and Apple buyer's tips at dealnews Price Trends: When to Buy a Mac