Is this a critical RAID error?
You have problems with drive in slot 1. It's RAID 5, so your data is protected, but you've lost redundancy (one disk is not reliable). Media error means the drive run out of spare sectors to remap bad sectors to (http://kb.lsi.com/KnowledgebaseArticle15809.aspx http://mycusthelp.info/LSI/_cs/AnswerDetail.aspx?inc=7468). If it was my data I'd be doubly scrupulous when backing up, remove the drive, replace it with a new one and synchronise the array. Some vendors (e.g. IBM) will accept RMA based on predictive failure indicators, some won't. If your vendor does not accept a disk with bad, un-remappable sectors as faulty, then take it out of the array and exercise in a test system. It should fail in reasonable time.
Edit:
Media events were non-zero only for disk with slot ID 1. In the log you've provided there's slot ID for each entry. The strange thing is, that the raid reports its state as optimal, despite media errors on the disk. Still', I wouldn't trust the disk.
RAID 5 made with n disks of the same size gives you capacity of (n-1) disks, because it stores one disks' worth of redundancy data. Therefore if you have six 250 GB disks and 1T of usable space, they are most likely divided into 5-disks RAID 5 (which gives you 4x250 GB of usable space) plus 1 spare disk.
actually smartctl can provide you detailed information about every disk in MegaRaid raid. to get information about physical disk #0 run:
smartctl -a -d megaraid,0 /dev/sda|less
as Pawel rightly points most probably it's reallocated sectors, but i had few cases when communication problems [visible in smartctl -l xerror -d megaraid,5 /dev/sda] were reported as Media Error Count.
As long as your array is up and running, it should be ok. Media error counter can increase from events such as a failing sector reallocation on one of the drives, while the other errors counter can be increased by any non-problematic event (bus device reset, power cycle, etc). However, if the error is critical, the drive will be automatically taken out of the array by the controller and reported as failed, in which case you will have to take an action.
It would be great if smartctl would be able to provide detailed SMART info on megaraid and individual unit status, but I don't think it supports it. Give it a try just in case.