Samba 3.5.5 is finally available for Solaris 10 in a supported way, SPARC patch 146363-01 and X86 patch 146364-01 updates the bundled Samba to version 3.5.5.
It should be available for download from support.oracle.com if you have a support contact. If you do not have that you could try out Solaris 11 Express which has the same version and also has native CIFS support in the kernel. A short summary of CIFS support in Solaris 8 through Solaris 11 Express presented at SNIA SDC 2010 can be viewed here: JiriSasek-CIFS_Map_Solaris.pdf.
Update: There is another patch available updating SAMBA to 3.5.8 with also fix at least one serious issue that prevents renaming of files on a ZFS filesystem. The new patch is 119757-20.
Solaris Samba update
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2 weeks ago