It shouldn't matter how often you get paid, although it is more difficult to get your head round how to budget at the beginning if you're not.
If you watch the tutorial videos on the website, you'll get a good feel for this as it's much more common in the US for people not to be paid monthly. It feels slightly uncomfortable as you either
- only budget for expenses that will occur before your next pay cheque, i.e. if you pay Council Tax on the last of the month, you don't try and budget it on 1 Jan if you only got a week's wages on 31 Dec.
- budget for a load of expenses you can't actually pay for using your available cash, and live with a load of red on your budget until you get paid closer to the date of the expenses falling due. e.g. you do budget [x] quid to your Council Tax and have YNAB tell you you've overbudgeted until you actually get paid again prior to the end of the month.
One of its great strengths is not being tied to any particular payment cycle - lots of people want to be able to budget from one pay cheque to the next one, let's say 16th of one month to 15th of another. But this can then leave you all over the place when the cycle changes - i.e. Christmas, change of job, etc.
As Living said, the goal is to get 'buffered', so you have enough money on hand to budget the whole month on day 1 of the month. If you're paid weekly or fortnightly as a lot of US YNABers are, this can be quite a challenge. If (like me) you're paid on almost the last day of the month, it's a piece of piss and the challenge then is to be truly buffered, which is not to budget December's pay cheque until February.
They are indeed working on a fully-featured iPad version - the owner of YNAB blogged recently about trying the beta version out so it's definitely coming.