Frank,
All 403(b) plans must have a written plan document. This is an Internal Revenue Code requirement found in the final 2007 403(b) regulations [26 CFR Section 1.403(b)-3(b)(3)]. It is not an ERISA requirement (although ERISA does have its own written plan requirement). Thus, you must have a written plan document even if the plan is not subject to ERISA. The plan document should have been adopted by the end of 2009. You may be able to satisfy the written plan requirement going forward by incorporating the terms of the annuity contracts or custodial accounts into something short of a formalized plan document that contains all the plan terms. However, you really should speak with an benefits attorney or consultant who has experience drafting 403(b) plans regarding your options.
You do not need to file Form 5500 unless your plan is subject to ERISA. Your plan is not subject to ERISA if it satisfies the "safe harbor" found in 29 CFR 2510.3-2(f) [available at:
http://www.dol.gov/dol/allcfr/title_29/Par...CFR2510.3-2.htm]. You should also take a look at Field Assistance Bulletin 2007-2 which addresses this issue [available at
http://www.dol.gov/ebsa/regs/fab2007-2.html]. Determining whether a plan is subject to ERISA is a fact-intensive inquiry and it is not always clear whether an employer satisfies the "safe harbor". In general, less employer involvement makes it less likely that the plan is subject to ERISA. If your plan is subject to ERISA, you will need to file Form 5500 every year even if no contributions are made, until the plan is terminated and all plan assets are distributed. You may want to file for an extension before July 31 to give you more time to sort this out.