I wrote last year about a startup offering a simple program that used cloud storage to trick your computer to behave as if it had infinite storage space. Now invites to a trial version of that program, Bitcasa, are starting to trickle out.

I was lucky enough to receive one and tried out both the Mac and Windows versions, of which the latter is described as “alpha” and seems not fully polished. But going by the experience of using the Mac version, Bitcasa is promising. I downloaded a 27 megabyte application and a few seconds later was being told by the Finder that I had a hard drive with over 500 terabytes of free space, an instant upgrade of more than three thousand times. In fact, Bitcasa will swallow as much data as you can push at it, I was told last year, but they weren’t able to hack an infinity sign (∞) into Mac OS.

Once you install Bitcasa it prompts you to choose which of your folders to “cloudify”. Cloudified folders are uploaded to Bitcasa’s cloud right away and get a Bitcasa logo added to their icons in the Finder. Any time you save, copy, or paste new files into a cloudified folder they also uploaded. The clever bit is what happens when you try and pile in more data than there is space for on your hard drive. Bitcasa arranges for some of your data to be stored only in the cloud, not on your PC, but it creates the illusion that all your files are stored locally. You can see them there using a file browser or a program’s open dialogue, but some files will be retrieved from the cloud when you try to open or access them.

That trick could make it much easier to manage a vast movie, music or photo collection larger than your hard drive, for example. Bitcasa could make them appear to be on your computer at all times so you could dig up an old song or photo with a few clicks. They’d take some time to download if you tried to open them, but it would be less hassle than using an external drive.

That design does create the capacity for you to be surprised by how long it takes to access a file, or even to find you can’t access all your data. Last year I heard their software would try and guess what you are most likely to need access to offline and make sure it’s stored locally. I haven’t used it long enough to tell if that’s the case for the beta version. My guess is that Bitcasa’s success will hinge on how well they can help users deal with a service designed to be forgotten about that very occasionally reminds you of its existence in a frustrating way, when you can’t access your data.

It adds up to a very different proposition to Dropbox, which is priced in a way that encourages you to use it to sync important files you need frequent access to, not for long-term backups (The personal version offers 2 GB for free, 50 GB for $9.99 a month and 100 GB for $19.99 a month). Bitcasa also requires less reorganizing of how you manage your data because of it’s neat integration with your computer’s file system, rather than requiring you to put stuff you want synced in a dedicated folder.

Bitcasa aren’t talking to the press right now, but they did confirm in a tweet last month that Bitcasa is free to use during the beta period and will cost $10 a month afterwards last month. Mobile versions appear to be in the works, going by seeing tweets about the company hiring mobile developers. Bitcasa’s Twitter feed also teased forthcoming “Big news for gamers coming soon” earlier in the week. They haven’t given any clues about how soon they will properly launch the service. Some beta users are being given invites to share, though. The first five people that click this link should receive one of my invites. If you get one, let us know in the comments how you liked Bitcasa.