Is it known an explicit formula for the number of subgroups of a given exponent of a finite abelian $p$group?
I had to look this up as well at some point in my research. The answer is yes, and a Google search for "number of subgroups of an abelian group" leads to several downloadable papers, not all of them easy to read. The paper "On computing the number of subgroups of a finite abelian group" by T. Stehling, in Combinatorica 12 (1992), contains the following formula and (I think) references to where it has appeared earlier in the literature. Let $\alpha = (\alpha_1,\dots,\alpha_\ell)$ be a partition, so that $\alpha_1\ge\cdots\ge\alpha_\ell$. (In this formula it is convenient to allow some of the parts of the partition at the end to equal 0.) Define the notation $$ {\mathbb Z}_\alpha = {\mathbb Z}/p^{\alpha_1}{\mathbb Z} \times \cdots \times {\mathbb Z}/p^{\alpha_\ell}{\mathbb Z} $$ for a general $p$group of type $\alpha$. Define similarly a partition $\beta$, and suppose that $\beta\preceq\alpha$, meaning that $\beta_j\le\alpha_j$ for each $j$. We want to count the number of subgroups of ${\mathbb Z}_\alpha$ that are isomorphic to ${\mathbb Z}_\beta$. Let $a=(a_1,\dots,a_{\alpha_1})$ be the conjugate partition to $\alpha$, so that $a_1=\ell$ for example; similarly, let $b$ be the conjugate partition to $\beta$. Then the number of subgroups of ${\mathbb Z}_\alpha$ that are isomorphic to ${\mathbb Z}_\beta$ is $$ \prod_{i=1}^{\alpha_1} \genfrac{[}{]}{0pt}{}{a_ib_{i+1}}{b_ib_{i+1}}p^{(a_ib_i)b_{i+1}}, $$ where $$ \genfrac{[}{]}{0pt}{}nm = \prod_{j=1}^m \frac{p^{nm+j}1}{p^j1} $$ is the Gaussian binomial coefficient. To answer your specific question, you'd want to sum over subpartitions $\beta\preceq\alpha$ such that $\beta_1$ equals the exponent in question. 


Greg's answer is a particular case of one of the many beautiful identities that arise in the study of Hall algebras http://en.wikipedia.org/wiki/Hall_algebra with full details in Ian Macdonald's Symmetric Functions and Hall Polynomials ISBN 9780198534891 http://ukcatalogue.oup.com/product/9780198504504.do 

