Glass-Steagall - an idea whose time has come (again)?

When the G20 leaders meet in London next week, a key item on the agenda will be how to reform worldwide regulation of the banking industry. They could do worse than take a look at the Glass-Steagall Act of 1933 (USA).
GSA was enacted in the aftermath of the Stock Market Crash of 1929 to separate commercial from investment banking activities. At the time it was thought that "improper banking activity, or what was considered overzealous commercial bank involvement in stock market investment, was deemed the main culprit of the financial crash" http://www.investopedia.com/articles/03/071603.asp
Sound familiar?
Commercial banks chafed at these restrictions, and with their eyes on the rich profits of their investment banking cousins eventually secured the repeal of the Act in 1999.
The basic principle for reforming regulation of the financial services/banking industries should be this: if the institution provides such services (i.e. taking deposits and making loans) and is of such a size that it can not be allowed to fail, then it should be tightly and boringly regulated as a commercial bank. Financial institutions which offer riskier products and services should be more lightly regulated, provided that it is made absolutely clear that they will not under any circumstances be rescued from the public purse. The principal regulation over these institutions, investment banks, should be a limit on their size (so that they do not become "too big to fail")