Take a look at RedSky Financial (www.redskyfinancial.com). While we do not make our own FOREX markets, we do offer api connectivity to Hotspot and Hotspot Institutional.
I believe you may be right and this is the deal with Oanda....
I think FXCM has an API as well.
If someone has better info than I do then please post the particulars.
Also forexsystembuilders on a yahoo group is working on an API there is one posted by Takeo there that is a work in progress. If you are a programmer this would be a good place to put some efforts.
We now offer RefcoPro prices and we are looking into Currenex. We also have EBS though one must have a Prime Brokerage account and counter-party credit. This is in addition to eSpeed, Hotspot Inst and Hotspot Retail.