Eurostar UK officially states that bookings can only be made 4 months (or 180 days) in advance. You are right though - it looks like on the US site, when using IE, you can make bookings all the way up until August 2008.

If booking from the US site, then your US debit card should be accepted. Eurostar accepts the following debit cards: Maestro, Switch, Visa Electron, Delta and Solo. If your booking is successful then you should receive a 6-letter PNR beginning with one of the following letters; Q, R, S or T. You can view full details of your booking at anytime on the Eurostar website by entering the email address you used for your booking and the 6-letter PNR.

Yes, day returns in Standard Class are cheaper than longer period returns on the US site. In the UK, however, day returns in Standard Class are priced exactly the same as a longer period return. Only First Class day returns are discounted.

If you are booking now for April 2008 then you should expect to be able to book the lowest fares. If you are doing a day return then you should book on the US site as the price, US$102, is slightly cheaper than the UK equivalent. If you intend on a longer period return then US$151 is not the lowest fare right now. I would suggest booking on the UK site in order to obtain the cheapest £59 return ticket. You will, however, have to wait until the beginning of December in order to book on the UK site for April next year.