I need a push

Although the Lumia 900 has served me well for just under a year now, the dust filled camera and lack of app support makes me yearn for something else. The replacement I had in mind is the Neuxs 4 (because its cheap-ish); but I need a couple questions answered before I drain my debit account.

1) Can I just pop my sim card from ATT into the Nexus 4? And will it work?

2) Is there any way I can unlock my Lumia 900 so it can be used on T-mobile (my friends Galaxy S1 is crapping out)

Bonus: Case reccomendations? Preferably 10-20 Dollars with free shipping

Bonus for the Bonus: Free shipping to Hawaii

Thanks for your future help!