I've been using BillMeLater for about 3 years now. They only did a hard pull for me the 1st time I used it. Every other time I have used it (last time was just a few days ago) they just do a soft pull to make sure that my credit didn't change drastically. Basically if you're approved the 1st time you try to use it, you will be approved for all future transactions unless your scores drop dramatically (say from 700 to 600). It's not exactly a credit account in that it does NOT show up on any of the CRs. And there is no pre-set credit limit, but if you read the explanation BillMeLater gives on their website, they determine how much credit they'll give you when they do a soft pull for each transaction.
This is all from personal experience, and maybe they haven't done any hard pulls (except the initial one) since I use this account a lot, but I thought it might be helpful info for other people.