More fascinating stuff on Islamic economics, from Mufti Taqi Usmani:

Moreover, when a company is financed on the basis of interest, its funds employed in the business are impure. Similarly, when the company receives interest on its deposits an impure element is necessarily included in its income which will be distributed to the share-holders through dividends.

However, a large number of the present day scholars do not endorse this view. They argue that a joint stock company is basically different from a simple partnership period. In partnership, all the policy decisions are taken by the consensus of all the partners, and each one of them has a veto power with regard to the policy of business. Therefore, all the actions of a partnership are rightfully attributed to each partner. Conversely, the policy decisions in a joint stock company are taken by the majority. Being composed of a large number of share-holders, a company cannot give a veto power to each share-holder. The opinions of individual share-holders can be overruled by a majority decision. Therefore, each and every action taken by the company cannot be attributed to every share-holder in his individual capacity. If a share-holder raises an objection against a particular transaction in an annual general meeting, but his objection is overruled by the majority, it will not be fair to conclude that he has given his consent to the transaction in his individual capacity, specially when he intends to withdraw from the income attributable to that transaction.

Therefore, if a company is engaged in a halal business, however, it keeps its surplus money in an interest-bearing account, wherefrom a small incidental income of interest is received, it does not render all the business of the company unlawful. Now, if a person acquires the shares of such a company with clear intention that he will oppose the incidental transaction also, and will not use that proportion of the dividend for his own benefit, how can it be said that he has approved the transaction of interest and how can that transaction be attributed to him? ...

Moreover, according to the principals of Islamic jurisprudence borrowing on interest is a grave sinful act for which the borrower is responsible in the Hereafter; however, this sinful act does not render the whole business of the borrower as haram impermissible. The borrowed amount being recognized as owned by the borrower, anything purchased in exchange of that money is not unlawful. Therefore, the responsibility of committing a sinful act of borrowing on interest rests with the person who willfully indulged in a transaction of interest, but this fact does not render the whole business of a company as un-lawful.

My quick take is that the anti-usury prescriptions in the Qu'ran are really, really difficult to get around, but modern-day scholars are doing their best to find loopholes, as Sistani does with foreign banks, and as Usmani seems to here.