According to a press release, if you sign up for a Softbank "4G" unlimited flat (4Gデータし放題フラット) plan by 4/30/2012, an up to ¥2,000 discount will be applied indefinitely. NTT Docomo is also offering a similar discount to customers who sign up for it's Xi LTE service by May, but unlike SBM, monthly service fees will increase to the standard ¥5,985, which is also the standard price of SBM "4G" unlimited flat.
However, unlike Xi, SBM's "4G" service is currently available only as a mobile wifi router. No phones are available. Compatible phones will certainly be released, but don't expect any until after this current campaign ends.
The standard discount is ¥1,000. Owners of SBM smartphones and tablets (excluding "X" series phones) may apply for an additional ¥1,000 discount, if they have one of the following data plans.
- パケットし放題フラット for スマートフォン
- パケットし放題MAX for スマートフォン
HOWEVER, from the end of October, similar to Xi, Softbank "4G" unlimited becomes UNLIMTED*: After 5 GB of usage, speed will be throttled to 128 kbps. Additional bandwidth will be available at ¥2,625 per each additional 2 GB. NTT Docomo will implement a similar plan with Xi, but the throttling is to begin from 7 GB.
NOTE: I always put the terms "4G" in quotes (or add an asterisk) to indicate that none of the networks purporting to be 4G actually are. The same often goes for term "unlimited," as in this post