Availability of next generation Macs?

So I'm selling my 8 core Windows 7 PC to fund the purchase of a new Mac when they are available. Based on past releases, what generally happens when a new Mac is announced?

Assuming the next generation of iMac and MacBook Pro are announced at WWDC, how long before they are available to buy on apple.com? Do they update the website overnight?

Are there usually shortages upon release? Should I be waiting at the computer to put my order through ASAP?

It seems like a great deal of people are waiting for these new systems too. I'm just not sure what to expect as this will be my first Mac purchase. Hopefully it's not like a Nikon/Canon DSLR release.