I like both of the answers that are posted above. Since there are many ways to interpret a poem, and since Cummings is especially open to personal interpretation, I'll offer my thoughts.

We usually think of love as an overwhelming experience, something that knocks us off our feet. A crush, an infatuation, or, most famously, a Romeo and Juliet type of love-at-first-sight. But Cummings has something else in mind with “somewhere i have never travelled,gladly beyond.”

The first time you read a poem, don't try to analyze it for anything. Just read it. In fact, read it a second time the same way. Then start looking for things—language and devices that the poet uses to convey his central message, his theme. One good way to analyze a poem is to look for diction patterns. Diction is word choice. Let's look at a common thread of diction that Cummings has chosen to include in this poem:

silence, frail, gesture, cannot touch, near, slightest, carefully, intense fragility, texture, breathing, small hands.

Cummings is using words like these to convey the idea that the love he is talking about is subtle. The object of his love is able to skillfully open and close his innermost being, like peeling back the petals of a flower, then closing them again. It's so much more than the explosive, all-enveloping experience that we normally call love. It is the ability of the loved one to open us up to something new, something we could not even see about ourselves.

Which brings us to the poem's last line (remember that this looks grammatically incorrect because Cummings writes this way on purpose):

nobody,not even the rain,has such small hands

Throughout the poem Cummings has referred to himself as a rose that can be opened and closed by his loved one. To take the metaphor a small step further, let's think about why Cummings mentions rain in the last line, for the first time in the poem. What do roses need to survive? Water, of course. In fact, once they have sprouted and grown, that's pretty much all they need. But water can also be deadly to something as delicate as a rose if it is not received in the correct quantity. Too much rain will kill any land-based plant. It takes the right amount. The rain's “small hands” are, as noted in a post above, individual raindrops. These raindrops are what the rose needs to flourish. The speakers love interest has something even better than the rain's small hands, in terms of her ability to affect him. It it the finer qualities in her eyes, her gestures, her slightest look, her intense fragility, that enables her to open and close him, to realize his true self, which is something he cannot do without her.

Cummings, a master of manipulating language to create unique poetic forms, most often wrote of love, childhood and flowers. He is known for the playful, cryptic way that he used words. This poem is no exception on either count. 

In this poem, Cummings speaks of falling in love, perhaps with someone who has no knowledge of his love. With lines such as "your slightest look easily will unclose me though i have closed myself as fingers, you open always petal by petal myself as Spring opens" he seems to be falling unexpectedly and perhaps reluctantly in love.

Throughout the poem he uses the symbol of her eyes and hands as the elements that draw her to his love. He compares his heart attempting to close but then opening to her love as a flower opening in spring. In the last lines, he expresses his wonder that she can so easily open his heart. With the line "nobody, not even the rain, has such small hands," Cummings expresses that only she can get to his heart and can do so more easily than the rain reaches a closed flower bud.  

This delicate poem works with small, fragile images to compare the narrator’s love to the listener; his experience with her is unique not because of great size but because it reaches him in small places never before experienced.  The early image, “in you most frail gesture are things which enclose me” we see the contradiction (a Cummings trademark), the use of unexpected comparisons.  The final line of the poem completes this comparison—even in something as small as a raindrop (and rain is a collection of small raindrops—“small hands”--into a soaking, overwhelming experience) I see how your every movement, every gesture, every flutter of your eyes is touching me, holding me in a million tiny embraces like raindrops in a rain, only smaller.  (Trying to paraphrase these images demonstrates how imaginatively and delicately Cummings puts words together in his poems.)