I was able to talk with someone who has knowledge of the upcoming b-mobile SIM U300 for cell phone data transfer. b-mobile will make a "best effort" to provide a 300 kbps data rate for burst transmissions, but any transfer that requires a sustained connection near the 300 kpbs limit will be throttled after a short amount of time, perhaps only a minute or two.
Activities such as browsing the web, sending an email, or using google maps should not impose any artificial cap on bandwidth. However, sustained use of a VOIP client or watching anything other than a very short clip on youtube will likely be impossible.
While this is disappointing, it is not surprising. b-mobile is an MVNO - that is they are buying bandwidth from Docomo and reselling it to their customers. In the comments section of the previous post on this topic, Gregconquest went into detail about potential pricing arrangements between Docomo and b-mobile for this and subsequent products. Suffice it to say that without bandwidth limits, b-mobile would not make a profit.