I purchased the CS4 Master Collection earlier in the year, PC license - that i use on my desktop computer
However i also have a MacbookPro, now i know that you cannot split a license between platforms - however due to this inability to cross-platform your purchase, can i buy a second package this year with a Mac license?
You're right that you can't go cross-platform with Creative Suite - you have to choose one platform or the other (PC or Mac)...
And according to Adobe's Student Edition FAQ, "Each eligible person may purchase only one copy per Adobe Student and Teacher Edition title" and "if you buy an additional copy ... no serial number will be issued for it."
However, there's nothing to say you couldn't purchase a CS5 version of your suite (e.g., CS5 Master Collection, which is a better product), or a slightly different flavor of a CS4 suite (e.g., CS4 Design Premium), although safe and legitimate copies of CS4 are increasingly hard to find.