From 1.401(a)-4, Q&A 2(b)(3) provides:
QUOTE
(3) Definition of "plan". For purposes of applying paragraph (a) of this Q&A-2, the term "plan" has the meaning that such term has for purposes of determining whether the amount of contributions or benefits and whether other benefits, rights, and features are nondiscriminatory under section 401(a)(4).
Because a SEP is not subject to 401(a)(4), I do not believe it can be considered for aggregate testing purposes.
Sammy, you mention (in your cross post)--
QUOTE
A footnote on page 474 of the Canan Qualified Retirement Plans book (2000 edition) states "See the discussion in Section 10.2 on meeting the requirements of IRC 401(a)(4) with respect to combined plans. "Plans" for this purpose includes simplified employee pensions, as discussed in Section 4.8." This is the only thing I have been able to find on this issue so I am looking for a more definitive answer.
Perhaps you can contact the author to see the above is still his/her current opinion?