A common method for dealing with non-flat geometries is the Proximity Force Approximation, which in effect assumes an effective Casimir interactions between locally parallel pairs on points on the two surfaces, and sums over all such pairs.
"Probing the Strong Boundary Shape Dependence of the Casimir Force,"
Emig, Hanke, Golestanian, & Kardar, Phys. Rev. Lett. 87, 260402 (2001)
Büscher & Emig, Phys. Rev. A 69, 062101 (2004) ("reduced distance" limit)
"Nonperturbative approach to Casimir interactions in periodic geometries,"
"Demonstration of the Nontrivial Boundary Dependence of the Casimir Force,"
A. Roy and U. Mohideen, Phys. Rev. Lett. 82, 4380 (1999)
"Measurement of the Casimir Force between a Gold Sphere and a Silicon Surface with Nanoscale Trench Arrays,"
H.B. Chan, Y. Bao, et. al , Phys. Rev. Lett. 101, 030401 (2008)
A sideways force to "align" the two plates at b=λ/2
"Mechanical Response of Vacuum," Golestanian, & Kardar, Phys. Rev. Lett. 78, 3421 (1997)
"Demonstration of the Lateral Casimir Force,"
F. Chen, U. Mohideen, et. al , Phys. Rev. Lett. 88, 101801 (2002)
Manipulating the critical Casimir force with chemically patterned surfaces:
"Critical Casimir forces in colloidal suspensions on chemically patterned surfaces,"
F. Soyka, O. Zvyagolskaya, C. Hertlein, L. Helden, & C. Bechinger, Phys. Rev. Lett., in press (2008) (movie)