I guess you distribute only the .lib, in this case it's up to you to ensure the CRT version.
Note that for 2008 SP1 there are two CRT versions, this one 9.0.30729.1 and this one 9.0.30729.6161, the latest is part of the MS11-025 hotfix, which is automatically installed in Windows 7 updates, BUT not on windows XP, so you have to make sure your client is using the same OS/stuff.
While it seams ugly, I personally always enforce CRT version when building, it's the only safe way I've found around years.

edit: but it doesn't seems to be your issue for now, maybe you even dont need shared stuff, if it breaks at link time its surely a MD/MT conflict as stated before.

I guess you distribute only the .lib, in this case it's up to you to ensure the CRT version.
Note that for 2008 SP1 there are two CRT versions, this one 9.0.30729.1 and this one 9.0.30729.6161, the latest is part of the MS11-025 hotfix, which is automatically installed in Windows 7 updates, BUT not on windows XP, so you have to make sure your client is using the same OS/stuff.
While it seams ugly, I personally always enforce CRT version when building, it's the only safe way I've found around years.