Sony Ericsson is going to change its old nomenclature of their entire product range. Some among you were surprised of the funky and weird namings of the latest Sony Ericsson products. This scheme will answer almost all of your questions regarding the new namings.
The future names won’t carry the typical “i” in the end of the product name. So the R300 and the R306 are the first phones that follow this new nomenclature. It isn’t really new, since we know the same nomenclature from the Japanese phones already. And here we go:

To get the whole story completed – a brief explanation of the upcoming series and their lower end parts:
• XPERIA vs. G-Series (G stands for: Generation Web)
The G-series is the lower end series compared to the XPERIA series. Both are determined as dedicated smart phone series.
• Cyber-shot – Series vs. Snapshot – Series (new S-series phones)
Cyber-shot has become a well known and well beloved series within the Sony Ericsson portfolio. It will get a new “sub-brand” called Snapshot containing new stylish phones in different form factors poorer equipped than the Cyber-shot models.
• Walkman – Series vs. Radio – Series (new R-series phones)
The smaller brother of the Walkman line will be the poorer equipped Radio line. It will be a nice extension regarding the music department of Sony Ericsson.
Some examples on existing phones:
• C902 – High-End Candybar (Cyber-shot)
• C702 – Mid-Range Candybar (Cyber-shot)
• R306 – Low-End Clamshell (Radio-Series)
Successors will be indexed for example C912… C922… C932, and so on 
As you can see, there are a lot of varieties and combinations… so SE won’t run out of new names that soon