How long are tablet batteries expected to last?

I notice that virtually all tablets have no access to the battery..(toshiba tablet allows battery access).
So, is there any info about battery life? I'm not referring to usage time after charged, but the total number of recharge cycles before an unacceptable hold time. With my woeful experience with notebooks...the battery life shortens pretty quickly and eventually requires constant recharging. At least with my phone and notebook I can buy and replace the battery myself.
I haven't seen any reference to this issue with sealed hardware.

Further to my previous post I had an email sent to me from a reader commenting how pleased they were with their battery life between charges. About 2 days on their xoom. (better than phone life). But they didn't know how long their battery would live in the long term.
I got to talking with a battery replacement specialist, and although he couldnt really give hard specifics he made some interesting comments.
Firstly he said that battery technology had improved significantly and lithium ion batteries were very robust and don't suffer the problems of nickel cadmium varieties which needed a special level of care. Although the manufacturers won't specifically say how many recharge cycles their batteries will take he would expect well in excess of a 1000 . So if you did a full recharge every day it might fail or experience problems after three years. (or more)

I figure on that basis, technology will have moved on and most people would be looking to replace or upgrade.