AT&T is trialling sales of a netbook for just $49.99. But the catch is that buyers must subscribe to a two-year internet plan which may wipe out any savings.

The deal is currently only available in Philadelphia and Atlanta but will be expanded if it’s a success. The lowest price machine offered is the Acer Aspire One, which comes with 1GB of memory and a 160GB drive – not too shoddy for an ultra-portable machine. Under the deal, it will cost $49.99, a $400 saving on the regular price charged by AT&T.

The deal is only available to customers who sign up for at least two years to a data plan. The cheapest available is $59.99, so you will be paying more for your first month’s service than for the machine itself. The way the numbers add up, you’d only be saving cash if you couldn’t find an equivalent or better data plan elsewhere for $43 a month or less.

It’s also worth noting this data plan isn’t particularly generous. It offers both a home DSL connection and mobile broadband. However, the DSL is only 768k, which barely qualifies as ‘broadband’. The mobile connection limits users to 200MB of data a month, meaning you won’t be able to do much more than occasionally check e-mails on the move.

Another option is to get a mobile data only plan (no DSL connection) which gives you a more generous 5GB data allowance, though that’s still not enough if you regularly stream video or play online games. (Insert “World of Warcraft/Never leaving the house” gag here.) This has the same monthly cost but puts the purchase price up to $99.99.

Such pricing plans are a growing, if fledgling trend. Radio Shack already runs a similar deal, with Verizon apparently working on such offers. At the moment it’s tough to see how most people would benefit: it might be useful for people on tight budgets who can’t afford to pay the full cost of a netbook upfront, though in those cases sixty bucks a month for a data plan might also be too high.

It might make sense to combine such deals with cellphone service so that users pay a single charge for both phone and netbook usage. However, the risk there is that anyone on more expensive phone packages probably has such a feature-packed smartphone that a netbook becomes virtually redundant.