Amazon's EC2 service offers a variety of Linux and Windows OS choices, but I haven't found a service offering a similar "rent by the hour" service for a remote Mac OS X virtual machine. Does such a service exist? (iCloud looks to be just a data storage service, rather than a service allowing remote login, etc.)
Such a virtual machine service would be very useful for testing software in a reproducible, "neutral" location.
Update 1: Just to be clear, I'm referring to services similar to EC2's on-demand or spot instances, where the machine (or virtual machine) is rented per hour, rather than typical web hosting services that involve a monthly subscription. As @Erik has pointed out, there are several good options for that route. As my searches for queries for OS X hosting with terms like "per hour" or "hourly rates" are turning up very little (basically, just labor fees for hourly repairs), I am inclined to believe that this doesn't exist for some reason. If it did, it seems reasonable that such a firm would advertise for precisely these queries.
Update 2: I see that this question is getting a lot of views over time. If someone encounters a change in the situation, i.e. that there is a provider of such services, please post and I will accept that answer instead.
I just came across this tonight. Can't say if they are legit, how long in business, and whether they'll be around long, but seems interesting. I may give them a try, and will post update if I do.
Per the website, they say they offer hourly pay-as-you-go and weekly/monthly plans, plus there's a free trial.
Per @Iterator, posting update on my findings for this service, moving out from my comments:
I did the trial/evaluation. The trial can be misleading on how the trial works. You may need to signup to see prices but the trial so far, per the trial software download, doesn't appear to be time limited. It's just feature restricted. You signup to get your own account, but you actually use a generic trial login account to do the trial, not your own account. Your own account is used when you actually pay for the service. The trial limits what you can do, install, save, etc. but good enough to give you an idea of how things work. So it doesn't hurt to signup to evaluate and not pay anything.
Persistence of data is offered via saving files to DropBox (pre-installed, you just need login/configure), etc. There is no concept of AMIs, EBS, or some VM image. Their service is actually like a shared website hosting solution, where users timeshare a Mac machine (like timesharing a Unix/Linux server), and I think they limit or periodically purge what you put on the machine, or perhaps rather they don't backup your files, hence use of DropBox to do the backup. One should contact them to clarify this if desired.
They have various pricing options, as you mention the all day pass, monthly plans at $20, and their is a pay as you go plan at $1/hr. I'd probably go with pay as you go based on my usage. The pay as you go is based on prepaid credits (1 credit = 1 hour, billed at 30 credit increments). One caveat is that you need to periodically use the plan at least once every 60 days for the pay as you go plan or else you lose unused credits. So that's like minimum of spending 1 credit /1 hour every 60 days.
One last comment for now, from my evaluation, you'll need high bandwidth to use the service effectively. It's usable over 1.5 Mbps DSL but kind of slow in response. You'd want to use it from a corporate network with Gbps bandwidth for optimal use. Or at least a higher speed cable/DSL broadband connection. On my last test ~3Mbps seemed sufficient on the low bandwidth profile (they have multiple bandwidth connection profiles, low, medium, high, optimized for some bandwidth ranges). I didn't test on the higher ones. Your mileage may vary.