It's fair to be concerned. Your biggest concern is probably technical support for your products, and that won't change, for now. All the people, products, and services in the IBM personal computer division are part of the deal, and this PC support infrastructure will supply tech support for IBM PCs and laptops for at least the next five years. So in the short term, don't expect drastic changes in support.
Long term, there's more cause to worry. The design and research-and-development arms of the companies will merge, as will the brands. Worst case: the features that make
ThinkPads the darlings of so many users will be diluted or even dropped as the joint venture aims to maximize revenues. Lenovo is acquiring IBM's ThinkPad development team as part of the deal, but there's no guarantee that it'll have the same goals under Lenovo as it did under IBM. The upside, though, is that future ThinkPads may be more cost-competitive than current models.
This answer has been updated from the original.
