The Sprint rebates, like I said are iffy, many people including me got them with SERO. The manf. rebates are:
$150 UTstartcom Manf. Rebate (used to be $100) this must be mailed in within 15 days of purchase, so I would recommend doing it regardless, no harm.
Also if you are still within your 15 day window take advantage of
Sprints $25 referral offer. You get $25 as well as the person who "referred" you. I'm sure you could contact anyone on this thread for a referral if needed.
$75 Kelloggs rebate (won't work for your price)
I think since you are on SERO that will make it all the more difficult since you may have to deal with InPhonic for returns as opposed to Sprint and they are horrible. Also SERO phones don't get updated as the same time as Sprint, so I think the 6800 will show up on SERO even later than it does on Sprint. I would recommend calling Sprint and finding out if you have to deal with Inphonic and then calling Inphonic to see what they say as well. Retentions might still do something for you of course even if your other options fail.