The best you can do for at&t is at&t premier - discounts for employees of certain companies and students from certain schools. Discounts range from about 15%-25% (maybe down to 10% in some instances) and the plans are all the same minus the discounts (at&t employees themselves only get 28% I believe). You won't find anything close to SERO on there, but if you have a school email address or work email address try entering it and see if it works, otherwise find a friend with one. Big companies like coke get discounts of 25%, my school (ASU) gives 15% so YMMV - try different emails and see which gives the best discounts. Discounts also vary for different parts (devices, service, etc) so play around and see what works best for you.
AFAIK smartphones are NEVER discounted, but the plans will be. Also IIRC data plans aren't discounted (but the voice part of your plan will still be) but this may only apply to iphone plans (fyi 3g iphone plans are discountable but not 2g iphone plans due to revenue sharing agreements with apple). Share the links with your friends that have at&t as many people that have at&t are not aware that their school/company offers discounts.
Please note that both of the links below will send an email which will require you to click a link so you can't just make up an email.
http://www.wireless.att.com/premier/
EDIT:
those with currently active at&t plans may add the discounts through this link:
http://www.att.com/wirelessdiscounts
note:
1) you cannot view the discounts
afaik from this link just add them so check your discount on the first link first.
2)
afaik this will reup your 2 year contract automatically so if you are out of contract or soon to expire it might be a bad idea. those who have just reuped due to purchasing a device and those who have just signed up will probably not care as it will just reset the contract length, not extend it a full 2 years.