The details of the dictionary support is not known yet, but it already seems to be only on the PRS-600. There are actually two "Oxford American" dictionaries for sale in the MOBI format:
New Oxford American Dictionary for $60 and
Concise Oxford American Dictionary for $19.95. The Kindle's all have the New Oxford American Dictionary, with over 250,000 entries and definitions. Probably the PRS-600 will have this version too (the Concise has 180,000 entries and definitions).
The price is one reason why this is probably missing from the PRS-300, although the lack of a touchscreen could be another. Obviously, Sony and Amazon are not paying $60 per device but they are paying something for the dictionary. I also doubt the PRS-700 will get a dictionary, based on the cost to Sony of adding this.
On the Kindles it is possible to add 3rd party dictionaries, but this has proven to be much more difficult than on other MOBI-based devices. I wonder if this will be possible at all on the Sony. Note that one issue will be DRM. Oxford University Press may be ok with providing a DRM-free dictionary if it is built into the firmware, but if so this will make commercial alternatives very unlikely.
Presumably, the dictionary will be different in each market.