This morning I've flashed my WRT610N.v2 with E3000 Ext (for Linksys E3000 ONLY!) and it running for 12 hours so far so good.
If something goes wrong with flashing of new CFE (BIOS of router) you will get totally bricked device without possibility to restore even with serial (the JTAG doesn't work on this device, as I know).
1) Converted my new router Linksys WRT610N.v2 to Cisco E3000 (following this or this guide). Only one difference is that I've used CFE.v20 from that post.
2) After flashing of dd-wrt.v24-14567_modified-e3000.bin I've reseted it to defaults and flashed with E3000 Ext b54.
I'll be glad to answer your questions.
p.s. Many thanks to teddy_bear for that divine firmware!