It appears that the Obopay/Citibank co-branded service is being shut down. From the sparse note on the webpage <citi.obopay.com> (see screenshot below), it doesn't sound like a temporary hiatus: 

As of Dec. 22, 2009, "Citi Obopay" will no longer be available.
If you would like to use the Obopay service go to www.obopay.com.
The service available at www.obopay.com has no affiliation with Citibank.

The companies have been working together on Obopay-powered mobile payment for more than three years.

Citi Mobile SVP Marylou Dowd said on Thursday in American Banker that the Obopay tests were concluded this summer. Not coincidently, Citi's mobile joint venture MMV announced last week that it was partnering with CPNI Inc. of Toronto to build a mobile P2P payments service.

Obopay's website still shows Citi Obopay on its Financial Institutions page (see inset above). But Citigroup, which invested at least twice in Obopay in the July 2007 C-round and April 2008 D-round, is not listed on Obopay's investors page.

The site was never publicized by Citibank. Traffic peaked at 7,000 unique visitors in Feb. before falling so low that Compete could not measure it after April 2009:

Source: Compete, 3 Dec 2009, link

For future reference, we've archived the website screenshots below.

Citi Obopay homepage (3 Dec 2009, 4 PM Pacific)
Note: The <citi.obopay.com> URL now redirects to the Obopay homepage.

How Citi Obopay Works page <citi.obopay.com>

Get Citi Obopay page <citi.obopay.com>

Citi Obopay Fees page <citi.obopay.com>

Note: For more info on mobile banking, see our Online Banking Report: Mobile Banking via iPhone (March 2009) as well as our earlier reports on Mobile Banking (Feb. 2007) and Mobile Payments (April 2007).