You can achieve directly what you want withmathtools' \mathclap. I also provide another variant that uses amsmaths \substack macro that gives you a multilined over-brace text (note also the \hfill to right-align the first line).
I also added a pair of braces {} between the + and the column separater & to get the right spacing around the plus sign. (This would be also necessary in \substack with the plus signs at the start of the lines, but since we're in script-style no horizontal space is added anyway.)
I get overfull boxes, by the way, so maybe you should split the equation onto different lines (see below).
Code
\documentclass[pdftex,a4paper]{scrartcl}
\usepackage[latin1]{inputenc}
\usepackage[left=2.5cm,top=2.5cm]{geometry}
\usepackage{mathtools}
\usepackage{amssymb}
\usepackage{bbm}
\begin{document}
\begin{align*}
\min \biggl(
\omega_1 \cdot \sum_{i \in \mathcal{F} } \sum_{c \in \mathcal{C} }
\sum_{e \in \mathcal{E}_c} \sum_{\tau \in \mathcal{S}^i_e}
\mathbf{w}^i_{c,e,\tau} \cdot x_{i,c,e,\tau} + {} &
\omega_2 \cdot \sum_{t \in \mathcal{T}} \sum_{c \in \mathcal{C}}
\overbrace{\mathbbm{1}_{\{ u_{t, c} \geq 0.8 \cdot \kappa^{\text{cap}}_c \}} (t,c) }^
{\mathclap{ \eta_{0.7} \mathbbm{1}_{\{ u_{t, c} \geq 0.7 \cdot \kappa^{\text{cap}}_c\}} (t,c)
+ \eta_{0.8} \mathbbm{1}_{\{ u_{t, c} \geq 0.8 \cdot \kappa^{\text{cap}}_c\}} (t,c)
+ \eta_{0.9} \mathbbm{1}_{\{ u_{t, c} \geq 0.9 \cdot \kappa^{\text{cap}}_c\}} (t,c)}
}
+ \omega_3 \cdot \frac{1}{2}\sum\limits_{t \in \mathcal{T}} \sum_{c \in \mathcal{C}} n_{t,c}
\biggr) \\
& \text{with} \quad \omega_i \in \left[0,1\right], \sum\nolimits_{i=1}^3\omega_i = 1
\end{align*}
\begin{align*}
\min \biggl(
\omega_1 \cdot \sum_{i \in \mathcal{F} } \sum_{c \in \mathcal{C} }
\sum_{e \in \mathcal{E}_c} \sum_{\tau \in \mathcal{S}^i_e}
\mathbf{w}^i_{c,e,\tau} \cdot x_{i,c,e,\tau} + {} &
\omega_2 \cdot \sum_{t \in \mathcal{T}} \sum_{c \in \mathcal{C}}
\overbrace{\mathbbm{1}_{\{ u_{t, c} \geq 0.8 \cdot \kappa^{\text{cap}}_c \}} (t,c) }^
{\mathclap{\substack{ \hfill\eta_{0.7} \mathbbm{1}_{\{ u_{t, c} \geq 0.7 \cdot \kappa^{\text{cap}}_c\}} (t,c) \\
+ \eta_{0.8} \mathbbm{1}_{\{ u_{t, c} \geq 0.8 \cdot \kappa^{\text{cap}}_c\}} (t,c) \\
+ \eta_{0.9} \mathbbm{1}_{\{ u_{t, c} \geq 0.9 \cdot \kappa^{\text{cap}}_c\}} (t,c)}
}}
+ \omega_3 \cdot \frac{1}{2}\sum\limits_{t \in \mathcal{T}} \sum_{c \in \mathcal{C}} n_{t,c}
\biggr) \\
& \text{with} \quad \omega_i \in \left[0,1\right], \sum\nolimits_{i=1}^3\omega_i = 1
\end{align*}
\end{document}
Output

No overfull boxes
\begin{multline*}
\min \biggl(
\omega_1 \cdot \sum_{i \in \mathcal{F} } \sum_{c \in \mathcal{C} }
\sum_{e \in \mathcal{E}_c} \sum_{\tau \in \mathcal{S}^i_e}
\mathbf{w}^i_{c,e,\tau} \cdot x_{i,c,e,\tau} \\ {} +
\omega_2 \cdot \sum_{t \in \mathcal{T}} \sum_{c \in \mathcal{C}}
\overbrace{\mathbbm{1}_{\{ u_{t, c} \geq 0.8 \cdot \kappa^{\text{cap}}_c \}} (t,c) }^
{\mathclap{ \eta_{0.7} \mathbbm{1}_{\{ u_{t, c} \geq 0.7 \cdot \kappa^{\text{cap}}_c\}} (t,c)
+ \eta_{0.8} \mathbbm{1}_{\{ u_{t, c} \geq 0.8 \cdot \kappa^{\text{cap}}_c\}} (t,c)
+ \eta_{0.9} \mathbbm{1}_{\{ u_{t, c} \geq 0.9 \cdot \kappa^{\text{cap}}_c\}} (t,c)}
}
+ \omega_3 \cdot \frac{1}{2}\sum\limits_{t \in \mathcal{T}} \sum_{c \in \mathcal{C}} n_{t,c}
\biggr) \\
\text{with} \quad \omega_i \in \left[0,1\right], \sum\nolimits_{i=1}^3\omega_i = 1
\end{multline*}

!in front of it to turn it into a link. A moderator or another user with edit privileges can then reinsert the!to turn it into an image again. – Qrrbrbirlbel Nov 04 '12 at 15:23