I recently explored the use of Secondlife banks. While searching I found out about JT Financial (secondlife link) which is a fairly popular inworld bank that pays the best rate I've seen so far. Below is a description of their bank/rates from their website: jtfinancials.com...
JT Financial offers our customers a sound banking system. We strive to offer a very competitive if not the best interest rate available on SecondLife along with a very supportive and friendly customer service. We can offer a very competitive interest rate because customer's funds are invested in a Real-World high-yield investment strategy that can provide us returns of .5% to 3% daily with very limited risks.
Setup of accounts is very easy. You simply pay one of their atms and your account is automatically created. You can check you balances by typing /10 balance and withdraw money by entering /10 withdraw AMOUNT where AMOUNT is the amount you wish to withdraw. You can do you banking anytime 24 hours a day.
So far I have I have received regular desposits of my interest daily from JT Financial as advertised. I think they run a really good system and service... whenever I've gone to the main bank, there are always customer service people there to help.
However, since there is no insurance of any kind, you could potentially lose some or all of your money. The continued interest and as well as your principle being preserved is dependent upon JT Financial's investments continuing to make money.
I believe the best strategy would be to spread your extra Lindens among several different respectable banks. I will be looking into other options to help diversify the risks.
When i first created my Secondlife account several months ago, I didn't have the best experience as my internet was slow and the grid had so much lag. So, I stopped using Secondlife and sold my original land plot. 
Jeremy Barkkorn is my name in