ohh, i see, u might have one of two problems
a) you havent installed the guest additions
-to install go into devices menu, and click install guest additions, this will prevent virtual box from having to 'capture' your mouse, and also add a bunch more features, full screen is the least of them*cough*Hostkey+L*cough* i need a doctor :[
b) u have guest aditions but havent enabled 'auto resize guest display, which makes your vboxs screen adjust to however you resize the window, even long and skinny if you want(i didnt mean it that way, but if u like that then thats cool

)
i luv vbox, i will let you discover the power of HostKey+L by yourself

, it doesnt work well in my version of ubuntu, 9.04. it works amaing though if you use a windows computer with windows also inside vbox
also, can i copy ur TEAM sig?