I have seen this issue back in SCCM 2007 but for the life of me I cannot find a reference to the issue.  I have a Windows 7 TS that has been working for days now but today out of the blue (or because of some change I made) the following happens:

I choose the OSD Task Sequence from the list after PXE booting...

...after resolving dependencies the TS downloads the boot image and prompts with:

Ready to Start
Remove the CD and do not boot from CD

<Name of Task Sequence>

Click Finish to start the task sequence

Does anyone recall how to resolve this issue?

Thanks

It came to me...Make all the boot images the same for all Task Sequences.

Good call ;-)

Erm any idea why only using a single boot image resolved the problem?

The PXE service point will instruct the client to use the boot image that's assigned to the task sequence with the highest advertisementID. The client will have to download the bootimage assigned to the task sequence if they do not match. Eliminating the first part will also eliminate the second one.